Senior Economics | Market Structure and Firm Behaviour Prep Guide (for tutors) | From MR = MC to deadweight loss in three steps

2026-11-06

Tutor lesson-prep guides · Secondary Economics

Students can say that monopolies charge more, but cannot draw marginal revenue below demand or find the deadweight loss triangle. This lesson uses one demand line and one marginal cost line to settle output, price and the efficiency loss in one go.

Objectives — Students can: (1) classify four market structures using number of firms, product differentiation and barriers to entry; (2) derive marginal revenue from demand and use MR = MC to find the profit-maximising output and price; (3) compare monopoly with perfect competition on price, output and total surplus, and calculate the deadweight loss.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Demand and supply	What does a downward-sloping demand curve mean	Lower price, larger quantity demanded
Marginal thinking	What does marginal cost measure	The extra cost of one more unit
Surplus	Which area is consumer surplus on a diagram	The triangle below demand and above price

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Opening diagnostic	Ask how a convenience store differs from a power company	Hands up
8-20 min	Four market structures	Classify by number of firms, differentiation and entry barriers	Fill a classification table
20-36 min	MR = MC sets output	Demand P = 100 – 2Q, MR = 100 – 4Q, giving Q = 20 and P = 60	Derive MR once themselves
36-50 min	Surplus and deadweight loss	Consumer surplus 400, deadweight loss 400	Compute the triangles
50-56 min	Efficiency and policy	Compare total surplus with perfect competition at Q = 40 and P = 20	State one policy implication
56-60 min	Exit check	Collect and mark three items	Work independently

03 Teaching script (read it out)

Scene 1: Three dimensions separate four structures

Classify with three dimensions: number of firms, whether products differ, and how high entry barriers are. Perfect competition has many firms, identical products and free entry. Monopolistic competition has many firms but differentiated products. Oligopoly has a few firms watching each other. Monopoly has one firm and very high barriers.
Make it concrete: a power company is a regulated monopoly, a convenience store sits in monopolistic competition, and vegetable stalls in a wet market come close to perfect competition.
The point of classifying is to predict behaviour: the fewer the firms, the more likely price sits above marginal cost.

Ask the student: which structure fits mobile network operators? Justify each of the three dimensions.

Scene 2: MR = MC: derive marginal revenue, then find the crossing

With demand P = 100 – 2Q, total revenue is TR = 100Q – 2Q squared, so marginal revenue is its slope: MR = 100 – 4Q.
Spell out the relationship: marginal revenue is always below price at the same quantity, because selling one more unit forces the firm to cut the price on every unit.
Set MC = 20 and solve MR = MC: 100 – 4Q = 20 gives Q = 20. Read the price off the demand curve: P = 100 – 2 x 20 = 60. Total revenue 1200, total cost 400, profit 800.

Ask the student: change MC to 40, recompute Q and P, and state which way output and price move.

Scene 3: Surplus and deadweight loss: two triangles

At monopoly price 60 and output 20, consumer surplus is half of (100 – 60) times 20, which is 400.
Under perfect competition P = MC = 20 and Q = 40, so consumer surplus is half of (100 – 20) times 40, which is 1600.
For the 20 units between 20 and 40, buyers value the good above marginal cost yet no trade happens. That triangle is the deadweight loss: half of (60 – 20) times (40 – 20) = 400.
Price elasticity fits neatly here: at the monopoly point the absolute elasticity is 1.5, so demand is elastic and a further price rise would cut revenue.

Ask the student: raise the monopoly price by 10, recompute the deadweight loss, and explain why it grows.

04 Question types and worked steps

Type	Example	Steps	Answer
Classify a market	Four local examples	Fill each of the three dimensions	Monopoly, oligopoly, monopolistic competition, perfect competition
Demand to MR	P = 100 – 2Q	Double the slope, keep the intercept	MR = 100 – 4Q
Output and price	MC = 20	Solve MR = MC, then read price off demand	Q = 20, P = 60
Deadweight loss	Compare monopoly with perfect competition	Half of price gap times quantity gap	400

Sanity check (teach this): Sanity check: the monopoly price 60 is above MC 20, as expected when the firm has market power; output 20 is below the competitive 40, the right direction. Consumer surplus falls from 1600 to 400; the 1200 difference equals the 800 transferred to the firm plus the 400 deadweight loss, so the accounts balance.

05 Independent practice (3 questions)

Question	Answer	What to watch
Classification, three items	Three local market examples	Were all three dimensions used
MR and MC, two items	Different demand and cost lines	Was the MR slope left un-doubled
Surplus and deadweight loss, three items	Different price and quantity pairs	Was the wrong base or height used in the triangle

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating MR as the demand line	Setting P = 100 – 2Q equal to MC directly	Only a perfectly competitive firm sets P = MC. A firm with market power uses MR = MC, and MR lies below demand.
Using MC = price for a monopoly	Solving Q = 40	That is the competitive output. The monopoly condition is MR = MC, giving 20 rather than 40.
Taking the whole triangle as deadweight loss	Writing half of 100 times 40	Count only the trades that fail to happen: half of the price gap times the quantity gap, not the whole consumer surplus.
Mixing profit with surplus	Calling the 800 profit the social surplus	Profit is revenue minus cost for the firm; surplus compares values with reservation prices. They are different concepts and cannot be added as total surplus.

07 Homework (paper, section and question numbers)

Secondary economics chapter index — All chapters at a glance
Prep guide master index — Compare similar chapters
Redo this chapter — Two MR = MC items and three deadweight loss items

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the MR when P = 100 – 2Q	100 – 4Q	Answering the demand line sends you back to act two
With MC = 20, what is the monopoly output	20	Answering 40 means the wrong condition was used
What is the deadweight loss in that case	400	Answering the whole triangle sends you back to act three

09 Tutor FAQ

Why is MR always below price?

Selling one more unit requires cutting the price on all earlier units, so the revenue lost on those units is subtracted from the new unit, leaving MR below price.

Is a monopoly always inefficient?

In the static diagram it creates a deadweight loss, but with large fixed costs or innovation a monopoly may be more efficient. Separate the assumptions from the conclusion instead of writing that monopolies are always bad.

Can the same formula be used for oligopoly?

Oligopoly is defined by interdependence between firms, so there is no single formula. Exams usually ask for behaviour such as price wars or collusion described with demand and supply tools, not a calculated equilibrium.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.