S5 Economics Practice Paper: Demand, Supply, Elasticity And Market Intervention (100 Marks)

S5 Economics Practice Paper: Demand, Supply, Elasticity and Market Intervention (100 marks)

2026-11-12

Economics – Markets and National Income

S5 Economics paper: 6 questions, 19 parts, 100 marks, 60 minutes. Demand and supply, elasticity, price controls, tax incidence and the national income multiplier.

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Part 1: Price Elasticity of Demand and Total Revenue

The price of a drink falls from 20 dollars to 16 dollars and daily quantity demanded rises from 100 cups to 120 cups.

#	Question	Answer and marking notes
1	(a) Calculate the price elasticity of demand using the arc formula and state whether demand is elastic, unit elastic or inelastic. (6 marks)	Answer: Average quantity is 110 cups and average price is 18 dollars. Quantity changes by 20 / 110 = 18.18 per cent, price changes by 4 / 18 = 22.22 per cent, so elasticity is 18.18 / 22.22 = 0.82. The absolute value is below 1, so demand is inelastic. Marking: formula 2, substitution 2, conclusion 1, sign or direction 1.
2	(b) Calculate total revenue before and after the price cut and state the change. (4 marks)	Answer: Total revenue before is 20 x 100 = 2000 dollars and after is 16 x 120 = 1920 dollars, so revenue falls by 80 dollars. Marking: each revenue 1, difference 1, direction of change 1.
3	(c) Explain the economic link between the elasticity value and the change in total revenue. (4 marks)	Answer: When demand is inelastic the percentage fall in price exceeds the percentage rise in quantity, so the price effect dominates and revenue moves with price. A price cut therefore lowers total revenue. Marking: elasticity below 1 stated 2, price versus quantity effect 1, conclusion 1.
4	(d) Should the firm cut the price further from 16 dollars if it aims to raise total revenue? Justify. (4 marks)	Answer: No. Demand is inelastic in this range, so a further cut reduces total revenue again. Raising the price would increase revenue, so the firm should raise price where costs allow and use non-price promotion. Marking: conclusion 1, elasticity reasoning 2, recommendation 1.

Part 2: Market Equilibrium and Price Controls

In a market, demand is Qd = 300 – 5P and supply is Qs = 100 + 3P, with quantity in thousand units and price in dollars.

#	Question	Answer and marking notes
5	(a) Find the equilibrium price and quantity. (5 marks)	Answer: Setting 300 – 5P equal to 100 + 3P gives 200 = 8P, so the equilibrium price is 25 dollars and the equilibrium quantity is 175 thousand units. Marking: equating 2, price 2, quantity 1.
6	(b) The government sets a price ceiling of 20 dollars. Find the shortage and explain how it arises. (6 marks)	Answer: At the ceiling, quantity demanded is 300 – 100 = 200 thousand and quantity supplied is 100 + 60 = 160 thousand, so the shortage is 40 thousand units. Price cannot rise, sellers do not expand output while buyers want more, so queues, rationing or black markets appear. Marking: demand and supply 2 each, shortage 1, process 1.
7	(c) If the government instead sets a price floor of 30 dollars, find the surplus and explain why the government often has to buy the excess. (6 marks)	Answer: At the floor, quantity demanded is 150 thousand and quantity supplied is 190 thousand, so the surplus is 40 thousand units. The price is above equilibrium, so buyers cut purchases while sellers expand output. Without government purchase, unsold stock forces output cuts or closures, so the government buys the excess to hold the price. Marking: demand and supply 2 each, surplus 1, reason 1.

Part 3: Unit Tax, Tax Incidence and Deadweight Loss

In a market, demand is Qd = 120 – 2P and supply is Qs = 4P – 60. The government imposes a unit tax of 6 dollars on sellers.

#	Question	Answer and marking notes
8	(a) Find the equilibrium before the tax, and the new price paid by buyers and the quantity traded after the tax. (6 marks)	Answer: Before the tax, 120 – 2P = 4P – 60 gives 180 = 6P, so price is 30 dollars and quantity is 60 units. After the tax the supply function becomes Qs = 4P – 84; 120 – 2P = 4P – 84 gives 204 = 6P, so buyers pay 34 dollars and 52 units trade. Marking: pre-tax 2, new supply 2, new price 1, quantity 1.
9	(b) Calculate the tax borne per unit by buyers and by sellers and state the shares. (5 marks)	Answer: Buyers pay 34 instead of 30, bearing 4 dollars, while sellers receive 28 instead of 30, bearing 2 dollars. Buyers bear 4 / 6 = 66.7 per cent and sellers 33.3 per cent. Buyers bear more because demand is less elastic than supply here. Marking: two tax burdens 1 each, shares 1 each, elasticity reason 1.
10	(c) Calculate the government tax revenue and the deadweight loss. (6 marks)	Answer: Tax revenue is 6 x 52 = 312 dollars. Deadweight loss is the triangle 0.5 x (60 – 52) x 6 = 24 dollars, the net loss of consumer and producer surplus caused by the fall in quantity traded. Marking: revenue 2, quantity gap 1, triangle formula 2, value 1.

Part 4: Cross and Income Elasticity

The price of good X rises from 40 to 50 dollars and the quantity of good Y rises from 200 to 240 units. Household income rises from 20000 to 24000 dollars; quantity of good Z falls from 100 to 90 units and quantity of good W rises from 50 to 65 units.

#	Question	Answer and marking notes
11	(a) Calculate the cross price elasticity of Y with respect to X and state whether the goods are substitutes or complements. (6 marks)	Answer: Quantity of Y rises 40 / 200 = 20 per cent and price of X rises 10 / 40 = 25 per cent, so cross elasticity is 20 / 25 = 0.8. The positive sign means the goods are substitutes, and a higher X price pushes buyers to Y. Marking: percentages 1 each, elasticity 2, interpretation 2.
12	(b) Calculate the income elasticity of Z and classify it as a normal or inferior good. (5 marks)	Answer: Income rises 4000 / 20000 = 20 per cent and quantity of Z falls 10 / 100 = -10 per cent, so income elasticity is -10 / 20 = -0.5. The negative value means Z is an inferior good. Marking: percentages 1 each, elasticity 1, classification 2.
13	(c) Calculate the income elasticity of W and classify it as a necessity or a luxury. (5 marks)	Answer: Quantity of W rises 15 / 50 = 30 per cent while income rises 20 per cent, so income elasticity is 30 / 20 = 1.5. A value above 1 means W is a luxury whose demand grows faster than income. Marking: percentages 1 each, elasticity 1, classification with reason 2.

Part 5: Cost Curves and Firm Decision

A firm has total cost TC = 400 + 20Q + 0.5Q^2 and takes the market price of 80 dollars as given.

#	Question	Answer and marking notes
14	(a) Derive the marginal cost function and find the profit-maximising output. (6 marks)	Answer: MC is the derivative of TC, so MC = 20 + Q. Profit is maximised where MC equals price, so 20 + Q = 80 and Q = 60 units, with MC rising at that output. Marking: MC 3, condition 2, output 1.
15	(b) Calculate total revenue, total cost and profit at that output. (5 marks)	Answer: Total revenue is 80 x 60 = 4800 dollars; total cost is 400 + 1200 + 1800 = 3400 dollars; profit is 4800 – 3400 = 1400 dollars. Marking: revenue 2, cost 2, profit 1.
16	(c) If the price falls to 15 dollars, should the firm shut down? Use average variable cost to explain. (5 marks)	Answer: Average variable cost is AVC = 20 + 0.5Q, with a minimum of 20 dollars as output approaches zero. A price of 15 dollars is below minimum AVC, so no output covers variable cost and the loss would exceed the fixed cost of 400 dollars; the firm should shut down and bear only fixed cost. Marking: AVC 2, minimum 1, comparison and conclusion 2.

Part 6: Equilibrium National Income and the Multiplier

A closed economy has consumption C = 200 + 0.75Yd, investment I = 300, government spending G = 400 and a lump-sum tax T = 200 with no transfers.

#	Question	Answer and marking notes
17	(a) Find the equilibrium national income. (6 marks)	Answer: Y = C + I + G = 200 + 0.75(Y – 200) + 300 + 400 = 750 + 0.75Y. Hence 0.25Y = 750 and equilibrium income is 3000 dollars. Marking: disposable income 2, substitution 2, solving 2.
18	(b) Find the multiplier and the new national income if government spending rises by 100 dollars. (5 marks)	Answer: The multiplier is 1 / (1 – 0.75) = 4. An increase of 100 dollars in government spending raises income by 400 dollars, so the new equilibrium income is 3400 dollars. Marking: formula 2, value 1, new income 2.
19	(c) Compute the government budget position and state whether it is a surplus or a deficit. (5 marks)	Answer: Tax revenue is 200 dollars and spending is 400 dollars, giving a net position of -200 dollars, so the budget is in deficit by 200 dollars. Marking: revenue 1, spending 1, net figure 2, conclusion 1.

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