ECO 101: Introduction to Microeconomics Exam Notes (South Africa)

Economics is the study of how people, firms, and governments make choices under scarcity. ECO 101 (Introduction to Microeconomics) focuses on the behavior of individual consumers and firms, and on how markets allocate resources through prices. These exam notes cover core microeconomic tools—demand and supply, elasticity, consumer choice, production and cost, market structures, and market failures—with a strong emphasis on the way these topics are typically tested in South African universities, colleges, and TVETs.

1) Microeconomics Foundations: Scarcity, Choices, and the Market System

Microeconomics examines decisions by households (consumers), businesses (producers), and governments in specific markets. In ECO 101, the emphasis is usually on building intuition with diagrams and then translating that intuition into precise statements and calculations.

What “Microeconomics” Means in ECO 101

At its core, microeconomics studies:

• Demand: how consumers respond to prices, income, and tastes.
• Supply: how firms respond to costs, input prices, technology, and regulation.
• Market equilibrium: how prices coordinate buyers and sellers.
• Efficiency and welfare: how outcomes compare to the idea of “social optimality.”
• Market power and structure: how monopolies, oligopolies, and monopolistic competition differ from perfect competition.
• Externalities and public goods: why markets sometimes fail.

In many South African exam papers, you’re asked to:

1. Interpret graphs (identify whether curves shift or move).
2. Explain the direction of change (e.g., “price rises because…”, “quantity demanded falls because…”).
3. Compute elasticity or use elasticity to justify tax/incidence outcomes.
4. Distinguish between types of costs (fixed vs variable; average vs marginal).
5. Evaluate market structures (price-taking vs price-setting; entry barriers).

Scarcity, Opportunity Cost, and Choice

A recurring theme is opportunity cost: the value of the next best alternative forgone when making a choice. Opportunity cost is not always financial; it’s often about time, resources, or risk.

Example scenario (everyday economics):
A student with limited budget chooses to spend R600 on textbooks and R400 on transport instead of buying a laptop for R1,000.

• If the laptop improves study productivity, its opportunity cost is the benefit of the R600 textbooks + the R400 transport enabling attendance and learning.
• This logic appears in exam questions involving trade-offs: “If government allocates funds to X, what is forgone?”

Exam-relevant distinctions:

• Explicit costs: direct monetary payments (rent, wages).
• Implicit costs: non-paid opportunities (using your own building for a business).
• Economic profit: total revenue minus economic costs (explicit + implicit).
  In contrast, accounting profit subtracts only explicit costs.

Production Possibilities and Efficiency

Many ECO 101 papers start with or reference the Production Possibilities Frontier (PPF). Even if not always calculated, PPF logic is often tested conceptually.

Key concepts:

• Scarcity means the economy cannot produce everything.
• Efficiency means producing on the frontier (not inside it).
• Inefficiency means producing inside the frontier (e.g., unemployment of resources).
• Economic growth shifts the PPF outward (more resources or better technology).
• Opportunity cost relates to the slope of the PPF.

Why it matters:
PPFs connect the “big ideas” to micro outcomes:

• Efficiency vs inefficiency becomes a baseline for judging market outcomes.
• Growth shifts possibilities similarly to technological improvements shifting supply.

Demand and Supply: The Market as a Coordination Mechanism

Microeconomics uses prices to coordinate decisions. The demand curve captures buyers’ willingness to pay and responsiveness; the supply curve captures sellers’ willingness to offer based on costs and profit motives.

Demand and supply are not “forces” like gravity; they are relationships:

• Demand curve: price ↔ quantity demanded (ceteris paribus).
• Supply curve: price ↔ quantity supplied (ceteris paribus).

In exam phrasing, you must be careful:

• A change in price moves along a curve.
• A change in non-price determinants shifts the curve.

Ceteris Paribus and Its Exam Importance

“Other things equal” is frequently used in diagrams and word problems. If non-price determinants change and you ignore that, you will describe the wrong graph.

Demand shifters (typical):

• Income (normal vs inferior goods)
• Tastes/preferences
• Prices of related goods (substitutes/complements)
• Expectations of future prices/income
• Number of buyers

Supply shifters:

• Input prices (wages, raw materials)
• Technology
• Taxes/subsidies
• Regulation
• Expectations
• Number of sellers
• Productivity/efficiency

Market Equilibrium and Comparative Statics

Equilibrium occurs where:

• Quantity demanded = quantity supplied.

In a basic model:

• If price is above equilibrium, quantity supplied exceeds quantity demanded → downward pressure on price.
• If price is below equilibrium, quantity demanded exceeds quantity supplied → upward pressure on price.

Comparative statics means you change one determinant and see how equilibrium changes, holding everything else constant.

Typical exam response structure:

1. Identify what changes (e.g., demand increases).
2. Draw or describe the shift direction.
3. Determine new equilibrium price and quantity.
4. Explain intuition (scarcity, willingness to pay, production incentives).

Case Study: Market for Textbooks in a South African College Setting

Consider a South African college where students commonly purchase textbooks for ECO 101 modules.

Suppose:

• A new edition becomes required for the next semester → student preference and perceived usefulness increases.
• Non-price determinant: tastes/requirements shift demand right.
• Equilibrium price rises and equilibrium quantity rises.

Now suppose:

• The college introduces a policy allowing bulk discounts to stores → retailers’ effective costs fall.
• Non-price determinant: input costs/technology (or purchasing power) shift supply right.
• Equilibrium price may fall while quantity increases.

These scenarios illustrate that outcomes can differ depending on whether demand shifts or supply shifts, and they prepare you for later exam questions involving taxes/subsidies and price controls.

2) Demand, Supply, and Elasticity: Graphs, Calculations, and Real Meaning

In ECO 101, many exam marks are awarded for correct interpretation of elasticity and correct diagram logic. Elasticity connects theory to policy: who pays when a tax is applied, whether consumers reduce purchases a lot, and whether a price change harms revenue.

The Law of Demand and Movements vs Shifts

The law of demand states: as price rises, quantity demanded falls, holding other things constant.

Movement along demand:

• Only price changes.

Shift of demand:

• Income/tastes/related prices/expectations/number of buyers change.

Exam trap: Students often say “demand decreases” when they really mean “quantity demanded decreases.”

• “Demand decreases” implies the whole curve shifts left.
• “Quantity demanded decreases” means you move down along the curve.

Market Demand and Individual Demand

Market demand aggregates individual demand at each price.

Example (simple aggregation):

• Individual A demands 10 units at R50.
• Individual B demands 6 units at R50.
• Market demand at R50 is 16 units.

In exams, if you’re given multiple demand schedules, you might be asked to compute market demand for a given price.

Determinants of Demand: Normal vs Inferior Goods

Normal goods: higher income → higher demand (right shift).
Inferior goods: higher income → lower demand (left shift).

South African relevance:
In many contexts, basic transport, low-cost brands, and cheaper staple products can behave as inferior goods when incomes rise and consumers upgrade to higher-quality alternatives.

But don’t guess—use the question data.
If an exam states “income rises and people buy less,” it signals an inferior good.

Price Elasticity of Demand (PED)

PED measures how responsive quantity demanded is to a change in price.

A common formula:
[
PED = \frac{%\Delta Q_d}{%\Delta P}
]

When using the midpoint method (often preferred in calculations):
[
PED = \frac{(Q_2 – Q_1)/( (Q_1+Q_2)/2)}{(P_2 – P_1)/( (P_1+P_2)/2)}
]

Interpretation Guide (Exam-Friendly)

• (|PED| > 1): elastic demand (quantity changes more than price).
• (|PED| = 1): unit elastic.
• (|PED| < 1): inelastic demand (quantity responds less).
• (|PED| = 0): perfectly inelastic (vertical demand).
• (|PED| = \infty): perfectly elastic (horizontal demand).

Always mention units/direction:
Since demand curves slope down, PED is usually negative. For exam interpretation, you often use absolute value (|PED|).

Elasticity and the Shape of Curves

• Steeper demand = more inelastic (in many standard diagrams).
• Flatter demand = more elastic.

However, for more precision, note:

• Elasticity depends on the specific price-quantity point, not only the slope.

Elasticity of Supply (PES)

PES measures responsiveness of quantity supplied to price changes.

Determinants (common):

• Time horizon (long-run supply tends to be more elastic).
• Ability to store inventory.
• Availability of variable inputs.
• Technology flexibility.

Time matters in exams.

• If the question says “short run,” supply is often more inelastic.
• If it says “long run,” supply can be more elastic.

Cross-Price Elasticity and Income Elasticity

Cross-Price Elasticity of Demand (XED)

[
XED = \frac{%\Delta Q_x}{%\Delta P_y}
]

• XED > 0: substitutes
• XED < 0: complements
• XED = 0: unrelated goods

Income Elasticity of Demand (YED)

[
YED = \frac{%\Delta Q}{%\Delta Income}
]

• YED > 0: normal good
• 0 < YED < 1: normal but necessities (sometimes called inelastic income response)
• YED > 1: luxury
• YED < 0: inferior good

Elasticity and Revenue

Total revenue (TR) is:
[
TR = P \times Q
]

Key exam logic:

• If demand is elastic, a price increase reduces total revenue (because quantity falls proportionally more).
• If demand is inelastic, a price increase increases total revenue (quantity falls less than proportionally).

Worked example style (for exam fluency):
Suppose price rises by 10% and quantity falls by 15%:

• PED = -15% / 10% = -1.5 → elastic in absolute value (1.5)
• Revenue: up in price but down in quantity; quantity effect dominates → TR decreases.

Taxes, Incidence, and Elasticity

Tax incidence is a common ECO 101 exam topic. Even though a tax is “paid” by someone in practice, the burden is shared depending on elasticities.

Core idea:

• The side with more inelastic demand/supply bears a larger share.
• The side with more elastic behavior can avoid the tax by reducing quantity more strongly or shifting to substitutes.

Diagram intuition:

• Tax creates a wedge between what consumers pay and what producers receive.
• Elasticity determines how steep the relevant curves are.

Example narrative consistent with micro logic:
Assume a tax on a commodity with few substitutes (inelastic demand). Consumers cannot easily stop buying it, so the price increases and consumers bear much of the burden. Producers also lose revenue but may pass less to consumers if supply is elastic.

Price Controls: Price Ceilings and Price Floors

Price ceiling (e.g., rent caps)

• Set below equilibrium price.
• Causes excess demand (shortage).

Price floor (e.g., minimum wage, minimum producer prices)

• Set above equilibrium price.
• Causes excess supply (surplus).

Elasticity can affect:

• Size of shortage/surplus.
• Deadweight loss magnitude.

Case Study: Electricity and Consumer Behavior (Policy-Relevant Interpretation)

Imagine an exam scenario about electricity tariffs:

• Households face higher electricity prices.
• In the short run, many households cannot quickly change appliances, so demand may be less elastic (more inelastic).
• In the long run, households can invest in energy-saving equipment, increasing elasticity.

An exam might ask:

• In the short run, who bears more of a tariff increase?
  Likely consumers because they can’t adjust quickly.
• In the long run, adjustment is greater, and supply/demand responses change.

You don’t need the specific tariff value unless given—but you must match the “short run vs long run” logic.

3) Consumer Choice and Firm Behavior: Utility, Budgets, Production, and Costs

This section connects “market prices” to decision-making. ECO 101 typically tests consumer optimality (the logic of utility maximization) and firm cost structure (how marginal cost relates to average cost and output decisions).

Consumer Choice: The Budget Constraint

A consumer has income (I) and faces prices for two goods, (P_x) and (P_y). The budget constraint:
[
P_x x + P_y y = I
]
Where:

• (x): quantity of good X
• (y): quantity of good Y

Graphically:

• The budget line slope is (-P_x/P_y).
• Intercepts: (I/P_x) and (I/P_y).

Changes:

• Income increase shifts the budget line outward.
• Higher price of one good rotates the budget line inward around the other intercept.

Exam-Style Interpretation Questions

• “If income increases and the consumer buys more of X, X is likely normal.”
• “If a higher price of X causes reduced purchases of Y, and Y is a complement, demand for Y falls.”

Preferences and Indifference Curves (Conceptual)

Indifference curves represent combinations of goods yielding equal utility.
Key properties:

• Downward sloping: more of one good requires less of the other.
• Convex to the origin: diminishing marginal rate of substitution (MRS).

Why it matters for exams:
Optimal choice in many standard frameworks occurs where:

• Budget line is tangent to the highest attainable indifference curve.

This tangency implies:
[
MRS = \frac{P_x}{P_y}
]

Utility Maximization: What the Tangency Condition Really Means

At the optimum:

• The consumer is willing to give up exactly the amount of one good needed to obtain more of the other, given relative prices.

Common exam wording:

• “At the optimum, the consumer maximizes utility subject to the budget constraint.”
• “Tangency means the rate of substitution equals the opportunity cost measured by prices.”

From Individual Choice to Demand

In micro theory, individual demand is derived from choice behavior. In ECO 101, you may not need full derivations, but you should understand the logic:

• A higher price changes the budget line.
• The consumer re-optimizes.
• The quantity purchased changes.

Consumer Surplus and Welfare Concepts

Consumer surplus (CS) is often described as:

• The difference between what consumers are willing to pay and what they actually pay.

On a demand-supply diagram (for a single price), CS is the area under demand above the price line.

Policy link:

• Price controls reduce welfare.
• Taxes reduce welfare.
• Externalities create welfare losses.

Even if the exam doesn’t ask for exact numerical areas, it may ask you to identify qualitatively how welfare changes when policies are introduced.

Firm Behavior: Production Functions and Inputs

A firm uses inputs (labor, capital, materials) to produce output. A production function can be described conceptually:
[
Q = f(L, K, \ldots)
]
Where:

• (Q): output
• (L): labor
• (K): capital

Total Product, Marginal Product, Average Product

These concepts frequently appear as calculations:

• Total Product (TP): total output at a given level of input.
• Marginal Product (MP): extra output from adding one more unit of input.
• Average Product (AP): output per unit of input.

If input is labor:

• MP rises initially, then falls (often due to diminishing marginal returns).

Exam tip:
When MP falls, AP may still be rising or falling depending on whether MP is above or below AP. If needed, remember:

• If MP > AP, AP rises.
• If MP < AP, AP falls.

Costs: Fixed vs Variable, Average vs Marginal

Firm costs divide into:

• Fixed cost (FC): does not change with output in the relevant range (e.g., rent).
• Variable cost (VC): changes with output (e.g., raw materials, wages paid per unit).
• Total cost (TC):
  [
  TC = FC + VC
  ]

Then:

• Average fixed cost (AFC): (FC/Q)
• Average variable cost (AVC): (VC/Q)
• Average total cost (ATC): (TC/Q)
• Marginal cost (MC): extra cost of producing one more unit.

Key relationship (often tested):

• MC intersects ATC at ATC’s minimum.
• MC intersects AVC at AVC’s minimum.
• MC is derived from changes in TC when Q increases.

Worked Cost Interpretation: How MC Behaves

In many standard micro graphs:

• MC is U-shaped due to diminishing marginal returns and then efficiency improvements.
• As output increases, variable inputs become less productive, raising marginal cost.

But the exact pattern depends on the question’s data. If the exam provides a cost table, rely on the table—not the generic U-shape.

Firm Decisions: Profit Maximization Basics

In a simplified competitive market:

• A firm aims to maximize profit:
  [
  \pi = TR – TC = (P \times Q) – TC
  ]

For profit maximization, a standard rule:

• Produce where marginal revenue (MR) = marginal cost (MC).

In perfect competition:

• The firm is a price taker, so (MR = P).

Thus:

• Choose output where (P = MC).

Example: Interpreting a Cost Table in an Exam

If an exam gives output levels and MC values, you might be asked:

• Find the profit-maximizing output.
• Compute profit at that output.
• Determine whether the firm is shutting down (depends on price relative to AVC).

Shutdown rule (conceptual but sometimes numerically tested):

• If price (P) is below AVC, the firm may shut down in the short run because it cannot cover variable costs.

Case Study: Small Business in a TVET Context

Consider a small catering business run by graduates from a TVET college. Inputs include:

• Labor (chefs/assistants)
• Ingredients (variable)
• Kitchen rent (fixed)

In micro terms:

• FC = kitchen rent and basic licenses.
• VC = ingredients and hourly labor.
• As the business produces more orders per day, initially labor specialization and planning may reduce average costs, but eventually congestion and ingredient constraints increase marginal costs.

If exam data show:

• MC rising steeply after Q = 80 meals, the firm’s optimal response might be to keep output near the point where MC aligns with market price—especially when facing competition and price taking.

4) Market Structures and Strategic Behavior: Perfect Competition to Monopoly

ECO 101 typically expects you to understand how firm behavior and outcomes differ under various market structures. Exams often ask for:

• Differences in number of firms and entry barriers
• Price-setting vs price-taking
• Demand curve shape faced by firms
• Implications for output, price, and profit

Perfect Competition

Characteristics:

• Many firms
• Homogeneous product
• Perfect information
• Free entry and exit
• Firms are price takers

Firm demand curve:

• Perfectly elastic (horizontal at market price (P)).

Profit and loss:

• In the short run, firms may earn economic profit or incur losses.
• In the long run, entry/exit pushes economic profit to zero (in the classic model).

Why this matters in exams:

• Perfect competition is used as a baseline for efficiency.
• It connects to welfare results like allocative efficiency.

Monopolistic Competition (Often Mentioned)

Characteristics:

• Many firms
• Differentiated products (branding, quality, location)
• Some market power due to differentiation
• Entry exists but not perfectly frictionless

Firm demand curve:

• Downward sloping (not perfectly elastic).

Exams might ask:

• Does the firm have MR < P? Yes, because the firm faces downward-sloping demand.
• Is there long-run zero economic profit? Often yes in the simplest model due to entry, but economic profit is not guaranteed at every time.

Oligopoly and Strategic Interdependence

Characteristics:

• Few firms
• Barriers to entry (capital costs, brand, networks)
• Interdependence: each firm’s decisions affect others’ outcomes.

In ECO 101, you might not compute complicated game theory, but you should know:

• Firms consider rival reactions.
• Pricing and advertising can be strategic.
• Collusion can occur (but is constrained by incentives to cheat).

Monopoly

Characteristics:

• One seller
• High barriers to entry
• Unique product (no close substitutes)

Demand curve faced by a monopolist:

• Downward sloping (not perfectly elastic).

Implication for MR and pricing:

• Because the monopolist can sell more only by lowering price, marginal revenue is less than price ((MR < P)).
• Profit maximization still occurs where:
  [
  MR = MC
  ]
• The monopolist then chooses a price from the demand curve (price-output relation).

Efficiency: Deadweight Loss and Welfare Trade-Offs

Monopolies can cause:

• Allocative inefficiency: output is typically lower than in competitive markets.
• Deadweight loss (DWL): total welfare lost.

Exams often connect this to:

• Elasticity of demand: monopolists consider responsiveness.
• Policy: regulation or antitrust may aim to improve outcomes.

Price Discrimination (Conceptual)

A monopolist may charge different prices to different consumers if it can segment markets and prevent resale. Conditions (classic):

1. Ability to identify different groups with different willingness to pay.
2. Prevention of arbitrage/resale.
3. Demand elasticity differs by group.

If priced well, discrimination can:

• Increase profits.
• Potentially increase consumer welfare in some cases.
• Still may reduce overall efficiency relative to perfect competition depending on assumptions.

Comparing Market Structures: A Consistency Table

Below is an exam-friendly summary of key differences:

Feature	Perfect Competition	Monopolistic Competition	Oligopoly	Monopoly
Number of firms	Many	Many	Few	One
Product type	Homogeneous	Differentiated	Often differentiated/homogeneous	Unique
Entry barriers	Low	Moderate	High	High
Price maker/taker	Price taker	Some price power	Strategic interdependence	Price setter
Demand curve faced by firm	Horizontal	Downward sloping	Downward sloping	Downward sloping
Profit in long run (basic model)	Zero economic profit	Zero economic profit (often)	Can persist	Can persist

Use this table to answer typical exam questions like:

• “Which market has a perfectly elastic demand curve for the firm?”
• “Which has MR = P?”
• “Which has high barriers to entry?”

Case Study: Market for Mobile Data in South Africa (Interpretive)

In many markets, real-world outcomes resemble oligopoly or monopolistic competition due to limited providers and differentiated offerings.

An ECO 101 question might describe:

• A few dominant telecom networks
• Pricing plans
• Brand loyalty and infrastructure barriers

Even if you do not have the formal labels from the exam, you can classify:

• Few firms → oligopoly features
• Differentiated plans and branding → monopolistic competition elements
• Strong network coverage and infrastructure → barriers to entry

If a question asks “why might pricing be sticky?”, the answer typically links to:

• Strategic interaction (oligopoly)
• Differentiation (limited substitutability)
• Switching costs and contracts

5) Market Failures, Government Intervention, and Exam-Ready Problem Solving

Microeconomics is not only about markets being efficient. ECO 101 also teaches why markets fail to deliver efficient outcomes and how government policy attempts to correct problems.

Externalities: When Costs/Benefits Spill Over

An externality occurs when a decision imposes costs or provides benefits to third parties not included in market prices.

• Negative externality: social costs exceed private costs (e.g., pollution).
• Positive externality: social benefits exceed private benefits (e.g., education).

Diagram intuition:

• For negative externalities: the supply (private cost) curve differs from social cost; equilibrium output is higher than the socially optimal output.
• For positive externalities: demand (private benefit) differs from social benefit; equilibrium output is lower than the socially optimal output.

Social vs Private Costs and Benefits

Often exam questions use the language:

• Private cost = cost paid by the firm/consumer.
• Social cost = private cost + external cost imposed on others.

Similarly:

• Private benefit = benefit received by buyer.
• Social benefit = private benefit + external benefit.

If asked:

• “Why does pollution lead to overproduction?”
  Because firms ignore external harm, so they understate the true social cost.

Taxes and Subsidies as Corrective Measures

For negative externalities:

• Pigouvian tax equals the marginal external cost.
• It raises private costs, shifting supply left, reducing output toward the socially optimal level.

For positive externalities:

• Subsidy equal to marginal external benefit.
• It increases private benefits, shifting demand right, increasing output toward social optimum.

Tradable Permits and Regulation (Conceptual)

An exam might ask about:

• Government sets a total pollution limit.
• Permits are allocated or auctioned.
• Firms can trade permits.

Tradeable permits can achieve:

• The same overall reduction but with flexibility.

For ECO 101, the exact institutional design may not require memorization, but you should understand:

• Market-based regulation uses price signals to allocate compliance cost.

Public Goods and the Free-Rider Problem

A public good has:

• Non-excludability: you can’t easily prevent people from consuming.
• Non-rivalry: one person’s consumption does not reduce availability for others.

Free-rider problem:

• People benefit without paying.
• Market demand for public goods may be insufficient, leading to under-provision.

Government intervention can correct this by:

• Tax funding
• Public provision
• Collective action mechanisms

Common Resources and Overuse

Common resources are rival but non-excludable (or costly to exclude). This leads to:

• Overuse because users do not bear full social costs.

A classic example:

• Fisheries, grazing land, or water resources.

In exams, you might be asked:

• Why do common resources get depleted?
• Link to open-access conditions: private incentives lead to over-extraction.

Asymmetric Information (Light Introduction in ECO 101)

Some ECO 101 courses briefly introduce:

• Adverse selection and moral hazard.

General intuition:

• If buyers cannot observe quality, low-quality sellers may dominate.
• If sellers can take actions after contracts are signed, they may cut corners.

Government responses:

• Regulation
• Certification
• Contracts and monitoring

Even if no deep calculations are needed, exam questions may ask:

• “How does asymmetric information cause market failure?”

Price Controls and Welfare Effects

Earlier we discussed price floors and ceilings. Here, connect them to welfare:

• Shortages/surpluses cause deadweight loss.
• Black markets may appear.
• Who benefits depends on elasticity and the type of control.

In exam explanations:

• Identify whether the policy creates a shortage or surplus.
• State that welfare loss arises because trades that would have occurred at equilibrium do not occur.

A Comprehensive Exam Problem-Solving Checklist

Because ECO 101 exams often blend conceptual and numerical questions, a consistent strategy improves performance.

Step 1: Identify the type of question

• Demand/supply shift?
• Movement along a curve?
• Elasticity computation?
• Cost/profit maximization?
• Market structure identification?
• Market failure correction?

Step 2: Write the “given” and “asked”

Create a short list:

• Given prices: (P_1, P_2)
• Quantities: (Q_1, Q_2)
• Costs: fixed/variable/marginal if provided
• Policy type: tax, subsidy, price ceiling/floor, regulation

Step 3: Use consistent micro language

• “Quantity demanded changes” → movement.
• “Demand changes” → shift.

Step 4: Choose the correct diagram and direction

• Demand shift right/left?
• Supply shift right/left?

Step 5: Calculate if needed

• Elasticity with midpoint method.
• Profit: (TR – TC).
• Welfare changes: qualitative or quantitative as required.

Step 6: Explain the intuition in one paragraph

Exams frequently reward explanation. Even when you have correct numbers, incomplete reasoning may cost marks.

Worked Elasticity Example (Midpoint Method)

Suppose:

• Price rises from R80 to R100.
• Quantity demanded falls from 50 units to 40 units.

Compute PED:

1. Percent change in quantity:
   [
   %\Delta Q = \frac{40-50}{(40+50)/2} = \frac{-10}{45/2} = \frac{-10}{45/2} = -\frac{20}{45} = -0.4444
   ]
2. Percent change in price:
   [
   %\Delta P = \frac{100-80}{(100+80)/2} = \frac{20}{180/2}=\frac{20}{90}=0.2222
   ]
3. PED:
   [
   PED = \frac{-0.4444}{0.2222} \approx -2.0
   ]
   Interpretation:

• (|PED| = 2.0) → elastic demand.

Revenue implication:
If price increases (as in the example) and demand is elastic, total revenue falls (quantity falls proportionally more).

Worked Tax Incidence Example (Conceptual with Numbers as Structure)

If an exam provides two elasticities:

• Demand is inelastic: (|PED| = 0.5)
• Supply is elastic: (|PES| = 2.0)

Then:

• Consumers’ quantity demanded doesn’t change much.
• Suppliers can adjust more (quantity supplied changes more).
• The burden tends to fall more on producers or consumers depending on who is more inelastic overall.
  A typical rule:
• More inelastic side bears more burden.

So in this case, demand is more inelastic (0.5) than supply (2.0), implying consumers bear more of the tax.

Cost and Shutdown Example

Suppose a competitive firm receives a market price (P). The firm has:

• Average variable cost at output is (AVC = R10)
• Market price is (P = R8)

Then:

• Since (P < AVC), the firm cannot cover variable costs.
• It would shut down in the short run.

If the question also provides fixed cost (FC = R100), the firm still might incur losses but the shutdown reduces losses relative to producing at a loss-making level where variable costs are not covered.

Profit Maximization Example (Using MR = MC in Perfect Competition)

If in perfect competition:

• (MR = P)

An exam may provide:

• MC schedule by output
• Market price (P)

Your task:

1. Find output where (MC = P)
2. Use that output to compute profit:
   [
   \pi = P \cdot Q – TC(Q)
   ]

Then interpret:

• Positive profit → economic profit if revenue exceeds economic costs.
• Zero economic profit can still allow accounting profit, depending on implicit costs (if included).

Case Study: Subsidizing Public Transport and Externalities

Public transport can generate positive externalities (reduced congestion, reduced emissions). A subsidy can increase usage toward the socially optimal output.

An exam might ask:

• Why is the market under-providing transport?
  Because private benefits are lower than social benefits.
• How does a subsidy help?
  It increases private incentives and shifts demand right (in the externality framework).

If the exam describes:

• Congestion and emissions are worsened when people drive private cars,
• Then driving has a negative externality; a subsidy to public transport indirectly corrects the imbalance.

South Africa-Focused Practice: Common Exam Themes Across Institutions

South African universities, colleges, and TVET campuses often teach ECO 101-style material with similar assessment patterns:

• Short conceptual questions: define elasticity, identify externalities, explain price ceilings.
• Graph-based questions: shift curves and interpret equilibrium changes.
• Numerical problem solving: elasticity using a given price and quantity change; profit from cost tables.
• Scenario writing: explain why a policy might raise prices but still reduce output or consumption.

Institution-agnostic but locally realistic contexts typically include:

• Consumer markets: food, transport, textbooks, internet bundles
• Firm markets: small manufacturing and services
• Policy contexts: taxes, subsidies, regulated prices, environmental regulations

In your exam responses, always anchor explanations in microeconomic mechanisms:

• Incentives (prices and costs)
• Constraints (budgets, entry barriers)
• Information (asymmetric information)
• Third-party effects (externalities)

Final Integrated Summary of What ECO 101 Exams Test Most

To score highly, master these “must-win” areas:

1. Demand and supply mechanics
   • shifts vs movements
   • equilibrium reasoning
2. Elasticity
   • calculations (prefer midpoint method)
   • revenue and tax incidence intuition
3. Consumer and firm fundamentals
   • budget constraint and optimization logic (conceptual tangency)
   • production and cost concepts (fixed/variable, average/marginal)
   • profit maximization basics
4. Market structures
   • perfect competition vs monopoly vs others
   • MR vs P relationship in different structures
5. Market failures and government policy
   • externalities (tax/subsidy)
   • public goods (free rider)
   • common resources (overuse)
   • how these policies change incentives and welfare

Exam-Ready Formula Sheet (Reference-Style)

Elasticity

• [
  PED = \frac{%\Delta Q_d}{%\Delta P}
  ]
• [
  XED = \frac{%\Delta Q_x}{%\Delta P_y}
  ]
• [
  YED = \frac{%\Delta Q}{%\Delta Income}
  ]

Budget Constraint

• [
  P_x x + P_y y = I
  ]

Costs and Profit

• [
  TC = FC + VC
  ]
• [
  \pi = TR – TC = (P \times Q) – TC
  ]

Shutdown Rule (Short Run, Competitive Firm)

• If (P < AVC), shutdown (do not produce).

Profit Maximization (Common Rule)

• Where [
  MR = MC
  ]
• In perfect competition: (MR = P), so choose output where (P = MC).

Quality Reminder for Diagrams (How Markers Usually Check Work)

When a question asks for a diagram, examiners typically assess:

• Correct curve used (demand vs supply vs social vs private).
• Correct direction of shift (right/left).
• Correct labeling of axes and equilibrium points.
• Correct new equilibrium (price and quantity change direction).

When a question is numeric, markers check:

• Correct formula choice.
• Correct substitution and arithmetic.
• Units/percentage signs when interpreting elasticity.
• Correct final interpretation (elastic vs inelastic, or profit vs loss).

If you want, I can also generate ECO 101 mock exam questions (with full solutions) aligned to these notes, including graph-drawing prompts and South African-style scenario questions.