S5 Economics Paper: Demand, Supply And Price Elasticity (100 Marks)

S5 Economics Paper: Demand, Supply and Price Elasticity (100 marks)

2026-11-12

S5 Economics: 6 questions, 24 parts, 100 marks, 75 minutes. Demand and supply, equilibrium, elasticity, tax incidence, price ceilings, floors and quotas. Full answers.

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Section A: Basic concepts of demand, supply and equilibrium (16 marks)

Source A: the bubble tea market in an estate mall. There were 8 shops at the start of the year and 12 at the end. Over the same period the district population rose by 6% while the import price of tapioca pearls rose by 25%. The average price was $20 per cup with 300 cups sold per day at the start, and $24 per cup with 240 cups sold per day at the end.

#	Question	Answer and marking notes
1	(a) Distinguish between demand and quantity demanded, and give one example of a change in each. (4 marks)	Demand is the whole schedule of quantities consumers are willing and able to buy at each possible price, drawn as the entire curve; quantity demanded is the amount bought at one particular price, a point on that curve. A change in the good own price changes only quantity demanded, a movement along the curve, while changes in other factors such as income, tastes, prices of related goods, expectations and population shift the whole curve. Example one: the price rises from $20 to $24 and daily sales fall from 300 to 240 cups, a fall in quantity demanded. Example two: population rises by 6%, so more is bought at every price and the demand curve shifts right, an increase in demand. Marking: 1 mark for each definition and 1 mark for each example.
2	(b) Does the rise in the number of shops from 8 to 12 change supply, demand or quantity demanded? Explain the effect on equilibrium price and quantity. (4 marks)	It is an increase in supply: the number of sellers is a non-price factor, so the supply curve shifts right by 50%. With demand unchanged the equilibrium price falls and the equilibrium quantity rises, because the extra supply puts downward pressure on price while the lower price attracts more purchases. Marking: 2 marks for identifying an increase in supply with a rightward shift, 1 mark for each of the price and quantity effects.
3	(c) If the 25% rise in the pearl import price outweighs the 6% rise in population, explain the changes in equilibrium price and quantity. (4 marks)	The dearer input raises production cost, so the supply curve shifts left, and the larger population shifts the demand curve right. Both shifts push the equilibrium price up, so the price definitely rises. The quantity effect depends on the relative size of the shifts: since the fall in supply is larger, the equilibrium quantity falls. Marking: 1 mark for each curve direction, 1 mark for the price conclusion and 1 mark for the quantity conclusion with reason.
4	(d) If the government imposes a specific tax of $3 per cup and demand is perfectly inelastic, who bears the tax? Explain. (4 marks)	The tax is borne entirely by consumers. With perfectly inelastic demand consumers buy the same quantity at any price, so the quantity stays at 300 cups, sellers pass the whole $3 on, the price paid by consumers rises by $3 and the price received by producers is unchanged. As the quantity does not change there is no deadweight loss, and tax revenue is $3 times 300 cups, that is $900 a day. Marking: 1 mark for the conclusion, 2 marks for the elasticity reason, 1 mark for the unchanged quantity and zero deadweight loss.

Section B: Calculating equilibrium price and quantity (17 marks)

In a market the demand function is Qd = 120 – 2P and the supply function is Qs = 3P -30, where Q is in thousands of units and P is in dollars. Both functions are written with P as the independent variable.

#	Question	Answer and marking notes
5	(a) Find the equilibrium price and quantity. (4 marks)	Set Qd = Qs: 120 – 2P = 3P -30, so 150 = 5P and P = $30. Substituting into demand gives 120 – 2 x 30 = 60 thousand units, that is 60,000 units. Checking supply: 3 x 30 -30 = 60 thousand units, the same value. Marking: 1 mark for setting up the equation, 1 for the price, 1 for the quantity, 1 for the check.
6	(b) If the government sets a price ceiling of $25, find the shortage and the quantity actually traded. (4 marks)	The ceiling of $25 is below the equilibrium price of $30, so it binds. Demand gives Qd = 120 – 2 x 25 = 70 thousand units and supply gives Qs = 3 x 25 -30 = 45 thousand units. The shortage is 70 – 45 = 25 thousand units, that is 25,000 units. The quantity traded is limited by supply to 45 thousand units, so 25 thousand units of demand go unsatisfied and queues, ballots or a black market may appear. Marking: 1 mark for each quantity, 1 for the shortage, 1 for the traded quantity and its consequence.
7	(c) If the market price is $40, find the surplus and explain how the market returns to equilibrium. (5 marks)	The price of $40 is above the equilibrium of $30. Qd = 120 – 2 x 40 = 40 thousand units and Qs = 3 x 40 -30 = 90 thousand units, so the surplus is 90 – 40 = 50 thousand units, that is 50,000 units. Stocks pile up, so sellers cut prices and production; the lower price raises quantity demanded and lowers quantity supplied until the price falls back to $30 with 60 thousand units traded, the market clears and the surplus disappears. Marking: 1 mark for each quantity, 1 for the surplus, 2 for the adjustment process.
8	(d) Calculate total revenue (TR) and consumer surplus (CS) at equilibrium. (4 marks)	TR = price x quantity = $30 x 60,000 units = $1,800,000. Consumer surplus is the triangle below the demand curve and above the price line. Setting Qd = 0 gives 120 – 2P = 0, so the price intercept is $60. CS = 0.5 x 60,000 units x ($60 – $30) = 0.5 x 60,000 x 30 = $900,000. Marking: 1 mark for TR, 1 for the intercept, 1 for the CS formula, 1 for the value.

Section C: Price elasticity of demand, calculation and application (16 marks)

Source B: a cinema raises its ticket price from $60 to $66 and daily admissions fall from 5,000 to 4,400. Source C: in another market the price of a skincare product rises by 12% and sales fall by 30%.

#	Question	Answer and marking notes
9	(a) Using the initial-value method, calculate the price elasticity of demand for cinema tickets and classify it. (4 marks)	Percentage change in price = (66 – 60) / 60 x 100% = 10% and percentage change in quantity = (4,400 – 5,000) / 5,000 x 100% = -12.0%. Price elasticity of demand = -12.0% / 10% = -1.2. Its absolute value of 1.2 exceeds 1, so demand is elastic: a ten per cent price rise cuts quantity by about twelve per cent. Marking: 1 mark for each percentage change, 1 for the elasticity, 1 for the classification.
10	(b) Recalculate using the arc method and explain why the two methods give different values. (4 marks)	Average quantity = (5,000 + 4,400) / 2 = 4,700 and average price = (60 + 66) / 2 = $63. The percentage change in quantity is (4,400 – 5,000) / 4,700 x 100% = -12.8% and the percentage change in price is (66 – 60) / 63 x 100% = 9.5%. The arc elasticity is -12.8% / 9.5% which is about -1.35. The difference arises because the initial-value method divides by the starting value while the midpoint method divides by the average, so with large changes the different denominators move the answer from -1.2 to -1.35. The midpoint method gives the same value whether the price rises or falls, so it is more reliable for large changes. Marking: 1 mark for the averages, 1 for the two percentage changes, 1 for the elasticity, 1 for the explanation.
11	(c) Calculate daily total revenue before and after the price rise, and use elasticity to explain the change. (4 marks)	Before the rise TR = $60 x 5,000 = $300,000 and after the rise TR = $66 x 4,400 = $290,400. The change is $300,000 – $290,400 = -$9,600, a fall of 3.2%. Because demand is elastic, with an absolute value of 1.2 above 1, the percentage fall in quantity exceeds the percentage rise in price, so the price rise loses too many customers and total revenue falls. The decision was unfavourable. Marking: 1 mark for each TR, 1 for the change, 1 for the elasticity explanation.
12	(d) Calculate the elasticity of the skincare product and advise whether the seller should raise or cut the price to raise revenue. (4 marks)	Elasticity = -30% / 12% = -2.5, with an absolute value of 2.5 above 1, so demand is elastic. With elastic demand a price cut raises quantity by a larger percentage than the price falls, so total revenue rises, while a price rise reduces total revenue. The seller should consider cutting the price, or holding the price and using non-price methods, to expand sales and revenue. Marking: 2 marks for the value, 1 for the classification, 1 for the advice with reason.

Section D: Income elasticity and cross price elasticity (17 marks)

Source D: monthly household income rises from $20,000 to $24,000 and sales of frozen food in a supermarket rise from 100 to 115 cases. Source E: the price of brand Y coffee rises from $40 to $44 per pack and sales of brand X tea bags fall from 200 to 180 boxes. Source F: when the price of brand Z soya milk rises by 8%, sales of brand X tea bags rise by 12%.

#	Question	Answer and marking notes
13	(a) Calculate the income elasticity of demand for frozen food. (4 marks)	Percentage change in income = (24,000 – 20,000) / 20,000 x 100% = 20.0% and percentage change in sales = (115 – 100) / 100 x 100% = 15.0%. YED = 15.0% / 20.0% = 0.75. Marking: 1 mark for each percentage change, 1 for the formula, 1 for the value.
14	(b) Using the value in (a), classify frozen food and explain what it means for the buying strategy of the supermarket. (4 marks)	YED is 0.75, a positive value, so frozen food is a normal good, and as the value is below 1 it is a necessity. Sales rise only modestly when income rises and fall only modestly when income falls, so demand is relatively stable. The supermarket can keep steady stock levels and long-term supply contracts instead of adjusting purchases sharply over the business cycle, and can focus promotion resources on goods with high income elasticity. Marking: 1 mark for normal good, 1 for necessity, 1 for stable demand, 1 for the buying strategy.
15	(c) Calculate the cross price elasticity between brand X tea bags and brand Y coffee and identify their relationship. (5 marks)	Percentage change in the price of Y coffee = (44 – 40) / 40 x 100% = 10.0% and percentage change in sales of X tea bags = (180 – 200) / 200 x 100% = -10.0%. XED = -10.0% / 10.0% = -1.0. The negative value means a higher coffee price lowers tea bag sales, so the two are complements: consumers buy them together, dearer coffee means less coffee and therefore fewer tea bags. With an absolute value of 1 the relationship is of unit strength, a moderate complement. Marking: 1 mark for each percentage change, 1 for the formula, 1 for the value, 1 for the identification and explanation.
16	(d) Using Source F, calculate the XED between X tea bags and Z soya milk, identify the relationship and compare it with (c) for pricing brand X. (4 marks)	XED = 12% / 8% = 1.5. The positive value means tea bag sales rise when soya milk becomes dearer, so the two are substitutes, and the absolute value of 1.5 is larger than the 1 in (c), so the substitution link is stronger than the complement link. For pricing, a price rise by Z soya milk pushes customers towards X tea bags, so brand X can promote more without cutting its price; if Z soya milk cuts its price, tea bag sales will come under clear pressure and brand X must hold customers through quality or bundle offers. Marking: 1 mark for the value, 1 for the identification, 1 for the comparison, 1 for the pricing implication.

Section E: Specific tax, tax incidence and deadweight loss (17 marks)

In a market the demand function is P = 70 – 0.5Q and the supply function is P = 10 + 0.5Q, where P is in dollars and Q is in thousands of units. The government imposes a specific tax of $10 per unit on sellers.

#	Question	Answer and marking notes
17	(a) Find the equilibrium price and quantity before the tax. (4 marks)	Setting 70 – 0.5Q = 10 + 0.5Q gives 60 = Q, so Q = 60 thousand units, and P = 70 – 0.5 x 60 = $40. Checking supply: 10 + 0.5 x 60 = $40, which agrees. Marking: 1 mark for the equation, 1 for the quantity, 1 for the price, 1 for the check.
18	(b) Find the price paid by consumers, the price received by producers and the quantity traded after the tax. (5 marks)	The specific tax shifts the supply curve up by $10, giving the new supply function P = 20 + 0.5Q. Setting 70 – 0.5Q = 20 + 0.5Q gives Q = 50 thousand units. Consumers pay P = 70 – 0.5 x 50 = $45 and producers receive $45 – $10 = $35. Marking: 2 marks for the new supply function, 1 for the quantity, 1 for the consumer price, 1 for the producer price.
19	(c) Calculate the share of the tax borne by consumers and by producers, and the total tax revenue. (4 marks)	Consumers bear $45 – $40 = $5, which is 10% of the $50 tax, and producers bear $40 – $35 = $5, which is 50%. Total tax revenue is the tax per unit times the quantity traded: $10 x 50,000 units = $500,000. Each side bears half because in this question the absolute price elasticities of demand and supply are equal, both 1.33. Marking: 1 mark for each share, 1 for the revenue, 1 for the elasticity explanation.
20	(d) Calculate the deadweight loss and explain why the burden is shared equally when the absolute price elasticities of the two curves are equal. (4 marks)	Deadweight loss = 0.5 x tax x fall in quantity = 0.5 x $10 x (60 – 50) thousand units = 0.5 x 10 x 10,000 = $50,000. It arises because the quantity traded falls from 60 to 50 thousand units, so 10 thousand mutually beneficial trades disappear. The split of the burden depends on how strongly each side responds to price: equal absolute elasticities mean consumers cut purchases by the same proportion as producers cut supply, so neither side can push the tax onto the other and the price rise equals the fall in the received price. Marking: 1 mark for the formula, 1 for the value, 1 for the lost trades, 1 for the explanation.

Section F: Price ceilings, price floors and quotas (17 marks)

In a market the demand function is Qd = 200 – 5P and the supply function is Qs = 10P -100, where Q is in thousands of units and P is in dollars. The government considers three measures: a price ceiling of $15, a price floor of $25 and a sales quota of 60 thousand units.

#	Question	Answer and marking notes
21	(a) Find the equilibrium price and quantity. (4 marks)	Setting 200 – 5P = 10P -100 gives 300 = 15P, so P = $20 and Q = 200 – 5 x 20 = 100 thousand units. Checking supply: 10 x 20 -100 = 100, which agrees. Marking: 1 mark for the equation, 1 for the price, 1 for the quantity, 1 for the check.
22	(b) If the government sets a price ceiling of $15, find the shortage and the quantity actually traded. (4 marks)	The ceiling of $15 is below the equilibrium price of $20, so it binds. Qd = 200 – 5 x 15 = 125 thousand units and Qs = 10 x 15 -100 = 50 thousand units. The shortage is 125 – 50 = 75 thousand units, that is 75,000 units. The quantity traded is limited by supply to 50 thousand units, so consumers may compete through queues, ballots or non-price means, and a black market may appear. Marking: 1 mark for each quantity, 1 for the shortage, 1 for the traded quantity and its consequence.
23	(c) If the government instead sets a price floor of $25, find the surplus and state the non-price competition sellers may use. (4 marks)	The floor of $25 is above the equilibrium price of $20, so it binds. Qd = 200 – 5 x 25 = 75 thousand units and Qs = 10 x 25 -100 = 150 thousand units. The surplus is 150 – 75 = 75 thousand units, that is 75,000 units. Since the price cannot be cut, sellers compete on non-price terms: better quality, free gifts, heavier advertising, longer warranties, and improved display and after-sales service. Marking: 1 mark for each quantity, 1 for the surplus, 1 for non-price competition.
24	(d) If the government sets a quota of 60 thousand units, find the price paid by consumers, the price received by producers and total quota rent, then compare a quota with a specific tax. (5 marks)	The quota of 60 thousand units is below the equilibrium quantity of 100 thousand units. The consumer price comes from the demand curve: when Qd = 200, P = (200 – 60) / 5 = $28. The producer price comes from the supply curve: when Qs = 60, P = (60 -100) / 10 = $16. Total quota rent is ($28 – $16) x 60,000 units = $720,000. Both measures keep the quantity below equilibrium, raise the consumer price, lower the producer price and create a deadweight loss. The difference is that the wedge from a specific tax goes to the government as tax revenue, while under a quota it becomes quota rent earned by whoever holds the quota if it is given away free, so government revenue is zero; the ownership of the rent is the key difference. Marking: 1 mark for each price, 1 for the rent, 1 for a similarity, 1 for the difference.

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