Demand and Supply

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Demand is the quantity of a good or service consumers are willing and able to buy at various prices during a period, other factors held constant (the ceteris paribus assumption). The Law of Demand states that, all else equal, quantity demanded falls as price rises — giving a downward-sloping demand curve. Supply is the quantity producers are willing and able to offer at various prices. The Law of Supply says quantity supplied rises with price — an upward-sloping curve. Market equilibrium price (Pe) occurs where Qd = Qs. Surplus arises when price is above Pe; shortage when price is below Pe. Price elasticity of demand is Ed = %ΔQd / %ΔP (absolute value). Watch for shift vs. movement traps in MCQs.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Demand Function and the Law of Demand

The linear demand function is typically written as Qd = a − bP, where a is the quantity intercept (when price is zero) and b is the slope (always positive in algebraic value, so the curve slopes downward). The Law of Demand relies on three pillars: the income effect (higher price reduces real purchasing power), the substitution effect (consumers switch to cheaper alternatives), and diminishing marginal utility (each extra unit consumed yields less satisfaction, so consumers pay less for additional units).

Supply Function and the Law of Supply

Supply is captured by Qs = c + dP, where c is autonomous supply (often zero for a single firm in the short run) and d is the slope. Producers respond to higher prices because of higher revenue per unit, the entry of new firms over time, and the ability to cover rising marginal costs. A change in input costs, technology, or the number of sellers shifts the entire curve; a change in the commodity’s own price causes only a movement along the curve.

Equilibrium Determination

Algebraically, set demand equal to supply and solve for P, then substitute back to find Q. Example: if Qd = 100 − 4P and Qs = 20 + 2P, then 100 − 4P = 20 + 2P → 80 = 6P → Pe = ₦13.33 and Qe = 46.7 units. Graphically, equilibrium is where the two curves intersect.

Shifts vs. Movements

A shift is caused by non-price determinants. For demand: consumer income, tastes, prices of substitutes (e.g. rice and garri) and complements (e.g. petrol and cars), expectations, and number of buyers. For supply: input prices, technology, taxes/subsidies, number of sellers, and producer expectations. A movement along the curve is triggered only by a change in the good’s own price.

Price Elasticity Essentials

Ed = %ΔQd / %ΔP is always reported as a positive absolute value: |Ed| > 1 elastic, |Ed| = 1 unit elastic, |Ed| < 1 inelastic. Total revenue moves with price only when demand is elastic.

Typical ICAN Question Patterns

Compute Pe and Qe from simultaneous equations.
Identify whether a diagram shows a shift or movement.
Determine whether a Nigerian market (e.g. cement, rice, fuel) is elastic or inelastic.
Explain the effect of a government price ceiling (creates shortage) or price floor (creates surplus).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Special Markets

Supply curves are not always upward sloping. In a perfectly competitive labour market, the backward-bending labour supply curve turns negative at high wage rates because higher income raises the demand for leisure. In agricultural markets, the short-run aggregate supply curve can be vertical because output is fixed by planting decisions already made. For perishable goods like tomatoes in Lagos markets, supply is almost perfectly inelastic in the very short run — a price collapse still leaves unsold inventory.

Elasticity Variants for ICAN

Beyond own-price elasticity, know three variants:

Income elasticity of demand (YED) — positive for normal goods, negative for inferior goods (e.g. pure water vs. sachet water substitution during income drops).
Cross-price elasticity (XED) — positive for substitutes, negative for complements.
Advertising elasticity — measures responsiveness to marketing spend.

Use the midpoint formula to avoid bias when computing % changes: %ΔQ = (Q2 − Q1)/((Q1 + Q2)/2) × 100.

Connections to Other ICAN Topics

Demand and Supply is the engine for foreign exchange determination in the Nigerian economy — naira depreciation reflects excess demand for dollars at official rates, leading to parallel-market premiums. Fuel subsidy removal shifts petrol supply right (lower cost to NNPC) but raises pump price, with elasticity determining how much quantity adjusts. Minimum wage policy is a price floor in the labour market — its employment effect depends on the elasticity of labour demand.

Common Mistakes to Avoid

Reading a rightward shift as a “movement along” because the price changed too.
Reporting elasticity with a negative sign instead of its absolute value.
Confusing normal and inferior goods with necessities and luxuries (these are different classifications).
Setting Pe equal to the y-intercept of the demand curve when the intercept is the maximum price consumers will pay, not the market price.

Practice Prompts

Given Qd = 200 − 5P and Qs = 40 + 3P, find equilibrium price, quantity, and the resulting consumer surplus.
The naira floats and the price of diesel rises 40%. If quantity demanded falls 10%, classify demand elasticity and recommend whether the federal government should expect a sharp drop in diesel revenue.

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Demand and Supply