S4 Economics: Production and Costs prep guide | Diminishing returns, fixed costs and economies of scale

2026-11-01

Tutor lesson-prep guides · Secondary Economics

Students can draw a U-shaped average cost curve but cannot say why diminishing returns pushes cost up. This lesson turns one five-worker table into every cost figure they need.

Objectives — Students can: (1) name the four factors of production and the gains from division of labour; (2) compute marginal and average product and identify where diminishing returns begin; (3) compute total, average, average variable and marginal cost and explain economies of scale.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Opportunity cost	The value of the best forgone option	The next best alternative
Averages	The average of 10 and 20	15
Marginal thinking	The extra output from one more unit	New total minus old total

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Four factors	Land, labour, capital and enterprise	One example each
10-24 min	Output table	Total product from 10 to 48	Marginal product [10, 14, 12, 8, 4]
24-36 min	Diminishing returns	Marginal product falls from 14 to 4	From the worker after the second
36-50 min	Cost figures	Fixed cost 400 and wage 200	Minimum average cost 27.27
50-56 min	Economies of scale	Specialisation lifts output by 37.5 per cent	22 against 16
56-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Factors of production and division of labour

Four factors: land for natural resources, labour for human effort and skill, capital for produced means of production such as machines and buildings, and enterprise for risk taking and organisation.
The gain from division of labour is a number here: two workers each doing both tasks make 16 units a day, while each specialising in one task makes 22, which is 6 more, or 37.5 per cent.
The reasons are practice, no time lost switching tasks and task-specific machinery. The limits are that the work must be divisible, the market must be large enough, and coordination and transport costs can bite.

Task: give one real division of labour from school or home, plus one limit.

Scene 2: Diminishing marginal returns

With one fixed kitchen and more workers, total product runs [0, 10, 24, 36, 44, 48], so marginal product is [10, 14, 12, 8, 4] and average product [10.0, 12.0, 12.0, 11.0, 9.6].
The second worker adds the most at 14 because specialisation starts to pay, then the third adds 12 and the rest add 8 and 4. That is the law of diminishing marginal returns: as more of one variable input is added to fixed inputs, marginal product eventually falls.
Both keywords matter, the other inputs staying fixed and the word eventually, because the rise at the start does not contradict the law. Once marginal product drops below average product, the average falls too, here from 12.0 to 9.6.

Task: explain why a rising marginal product at the start does not break the law.

Scene 3: From the output table to cost curves

Fixed cost is 400 for rent, and each worker costs 200, so variable cost is [0.0, 200.0, 400.0, 600.0, 800.0, 1000.0] and total cost [400.0, 600.0, 800.0, 1000.0, 1200.0, 1400.0].
Average cost is total cost over output, giving [60.0, 33.33, 27.78, 27.27, 29.17] with a minimum of 27.27 at 4 workers and 44 units. Marginal cost is the change in total cost over the change in output, giving [20.0, 14.29, 16.67, 25.0, 50.0], so the fourth worker adds 25.00 a unit.
That minimum average cost of 27.27 is the long-run break-even price under perfect competition, where price equals minimum average cost and profit is normal.
Economies of scale push long-run average cost down through technology, bulk buying and cheaper finance, while diseconomies come from too many management layers and coordination failure.

Warn: marginal cost divides by the change in output, never by the number of workers.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Marginal and average product	Change in output over change in labour	[10, 14, 12, 8, 4]
Calculation	Total and average cost	Fixed plus variable, then divide by output	Minimum 27.27
Calculation	Marginal cost	Change in total cost over change in output	25.00
Short answer	Economies of scale	Name three sources plus a diseconomy	Management overload

Sanity check (teach this): Total cost is still 400 at zero output, which proves the fixed cost is there. The minimum average cost of 27.27 must sit close to marginal cost 25.00, so a wide gap means a division error. The marginal products must add to the final total: 48 equals 48. Average variable cost 16.67 must be below average cost 27.78, because the latter includes fixed cost.

05 Independent practice (3 questions)

Question	Answer	What to watch
Marginal product of the third worker	12	36 minus 24
Total cost with three workers	1000	400 plus 600
Average cost with three workers	27.78	1000 over 36
Marginal cost from the third to the fourth worker	25.00	200 over 8
Minimum average cost and its output	27.27 at 44 units	Compare all five rows

06 Common mistakes and what to say

Mistake	Student writes	Say this
Dividing by workers	Answering 200 instead of 25.00	“Marginal cost is per extra unit, so divide by extra output.”
Dropping the fixed cost	Writing total cost as 600	“Rent is paid even at zero output, so add fixed cost.”
Forgetting the conditions	Saying only that output falls	“Say other inputs are fixed, and say eventually.”
Mixing up specialisation and scale	Calling division of labour an economy of scale	“Division of labour is about inputs; economies of scale are about long-run average cost.”

07 Homework (paper, section and question numbers)

Secondary economics chapter index — All chapters
DSE economics practice paper — Firm and production questions 8 to 13

08 Exit check (last 5 minutes)

Question	Answer	Decision
Marginal product of the fourth worker	8	All correct: move to market structure
Average cost with three workers	27.78	Wrong: redo the cost table
The two keywords for diminishing returns	Other inputs fixed, and eventually	Two wrong: restart act two

09 Tutor FAQ

Why does marginal cost cross average cost at its minimum

Below the average it drags the average down, above it pushes the average up, so the crossing is the minimum.

Does fixed cost affect marginal cost

No, because it does not change with output, so only variable cost enters the change.

Is diminishing returns the opposite of economies of scale

No, diminishing returns is short run with fixed capital, while economies of scale is long run with all inputs expanded.

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