How does a unit tax affect a monopoly price?

Marginal cost rises, output falls and price rises by less than the tax.

Why is a natural monopoly regulated?

Economies of scale make one producer cheapest, but it would charge a high price.

How does markup pricing relate to elasticity?

The lower the elasticity, the higher the markup.

How does a merger affect concentration?

The index rises, so concentration and market power increase.

S5 Economics paper 3: taxation, regulation and mergers

2026-11-10

Tutor Prep Guide · S5 Economics Monopoly Regulation

Ten items cover the Lerner index, pass-through of a unit tax, consumer surplus, deadweight loss, natural monopoly, changes in the Herfindahl index and markup pricing.

Direct answers　Items 01 to 03 cover marginal and average cost, monopoly pricing and the Lerner index, 04 to 06 tax pass-through, consumer surplus and deadweight loss, 07 to 09 regulation of a natural monopoly, the index after a merger and markup pricing, and 10 the use of a rectangle for deadweight loss.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S5 Economics Market Structures and Monopoly
Level	S5

02　Questions (10 items)

FoundationTotal cost is TC = 500 plus 4Q plus 0.25Q squared. Find marginal and average cost at Q = 40.
FoundationDemand is P = 120 minus 3Q and marginal cost is fixed at 60 dollars. Find the profit-maximising output and price.
IntermediateWith a price of 90 dollars and marginal cost of 60 dollars, find the Lerner index and the implied elasticity.
IntermediateA unit tax of 12 dollars raises marginal cost from 60 to 72 dollars. Find the new output, price and the share of the tax passed to consumers.
ChallengeFrom question 2, marginal cost pricing gives 20 units at 60 dollars. Find consumer surplus under competition and under monopoly, and the loss.
ChallengeFrom question 2, find the deadweight loss caused by the monopoly.
ChallengeA natural monopoly produces 100 units with average cost of 50 dollars and marginal cost of 30 dollars. Find the loss under marginal cost pricing and explain the policy issue.
FoundationFive firms hold 30, 25, 20, 15 and 10 per cent. If the top two merge, find the change in the Herfindahl index.
IntermediateElasticity is 4 and marginal cost is 60 dollars. Find the markup price and the markup rate.
Error redoA common slip: a student computes deadweight loss as 30 times 10, or 300 dollars. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	MC is 4 plus 0.5Q, or 24 dollars; AC is 1,060 divided by 40, or 26.5 dollars.	Marginal cost below average cost means average cost is still falling.
2	Q is 10 and P is 90 dollars (120 minus 6Q equals 60).	With constant marginal cost, output follows from marginal revenue equal to marginal cost.
3	The index is one third, or 33.3 per cent, which implies an elasticity of 3.	The Lerner index equals the reciprocal of the elasticity.
4	Q is 8 and P is 96 dollars; price rises 6 dollars, so 50 per cent is passed on.	With linear demand and constant marginal cost, half of the tax is passed on.
5	Competition gives 600 dollars and monopoly gives 150 dollars, so 450 dollars of surplus is lost.	Both triangles are bounded by the demand curve and the price line.
6	The deadweight loss is half of 30 times 10, or 150 dollars.	Only the surplus lost on the 10 units not produced counts.
7	Pricing at 30 dollars loses 20 dollars a unit, or 2,000 dollars in total, so a subsidy or average cost pricing is needed.	Marginal cost lies below average cost, so marginal cost pricing always makes a loss.
8	The index rises from 2,250 to 3,750, an increase of 1,500.	A merger inside the same market raises the index.
9	The price is 60 times four thirds, or 80 dollars, a markup of 33.3 per cent.	The markup rate is one divided by elasticity minus one.
10	The correct figure is the triangle, half of 30 times 10, or 150 dollars; the rectangle doubles the loss.	The loss covers only the units not produced, not the whole output.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Book an Economics tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.