DSE Economics | Firm Growth and Mergers Tutor Prep Guide | Three merger types, economies of scale and HHI

2026-11-05

Tutor lesson-prep guides · Secondary Economics

Students answer every merger question with the phrase bigger and more profitable, while the marks sit in classification, motive and market concentration. Two questions classify any case, and one calculation settles the concentration argument.

Objectives — Students can: (1) classify any merger as horizontal, vertical or conglomerate using the stage question and the market question; (2) explain economies and diseconomies of scale and compute the 20 percent cost saving from an average cost falling from 40 to 32; (3) compute HHI for shares [30, 25, 20, 15, 10] as 2250, and 2850 after the merger, and explain what the rise means.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Cost concepts	How do average and marginal cost differ?	Total cost divided by output versus the cost of one more unit
Market structure	What separates perfect competition from oligopoly?	The number of firms and pricing power; an oligopoly has a few interdependent firms
Market share	How is market share computed?	Firm sales divided by total market sales, times 100 percent

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Prior-knowledge check	Ask the three prior questions, write wrong ones on the board	Answer orally, copy the wrong one
5-17 min	Act 1: two questions sort three types	Ask about stage and market for each of three cases	Classify three cases and justify
17-33 min	Act 2: economies and diseconomies	Compare average cost at 1000 and 2000 units	Compute the percentage saving
33-42 min	Act 3: HHI before and after	Compute 2250 and 2850, explain the rise of 600	Recompute and write one conclusion
42-43 min	Redo the worst items	Reteach only the two most-missed items	Correct in red and write one sentence
43-45 min	Exit check	Ask three exit questions and record who misses	Answer orally, note for next lesson

03 Teaching script (read it out)

Scene 1: Act 1: two questions classify everything

First question: are the two firms at the same stage of production? If yes it is a horizontal merger, such as two tutorial centres or two bakeries.
Second question: are they at different stages of the same chain? A flour mill buying a bakery is a vertical merger, upstream into downstream.
If both answers are no, it is a conglomerate, such as a bakery buying an insurer.
Common misconception box: students treat same industry as horizontal. Same chain but a different stage is still vertical.

Ask students to repeat: ask about the stage first, then the market.

Scene 2: Act 2: economies of scale push average cost down

Output rises from 1000 to 2000 units while average cost falls from 40 to 32. Total cost rises from 40000 to 64000, yet cost per unit falls by 20 percent.
Name the four sources: specialisation, bulk buying, indivisible capital and managerial division. All four mean the same thing, larger output means lower cost per unit.
The reverse is diseconomies of scale: too many management layers and slower communication push average cost back up, which is why long-run average cost is U-shaped.
Common misconception box: students expect total cost to fall after a merger. Total cost usually rises; what falls is cost per unit.

Ask students to repeat: economies of scale are about average cost, not total cost.

Scene 3: Act 3: HHI turns shares into concentration

Square each market share and add: the squares of [30, 25, 20, 15, 10] sum to 2250.
The firms with 20 and 15 merge into 35, and the new sum is 2850, a rise of 600.
The conclusion has three parts: before 2250, after 2850, and concentration has risen so the market is closer to oligopoly, price competition may weaken and a regulator will review it.
Common misconception box: students add shares and call the total HHI. Shares must be squared because large firms then weigh disproportionately.

Ask students to repeat: HHI is a sum of squares and must be recomputed after a merger.

04 Question types and worked steps

Type	Example	Steps	Answer
Classification	A bakery chain buys a flour mill. Which type of merger is this?	Ask the stage question, ask the market question, conclude	Vertical, upstream into downstream
Cost calculation	Output rises from 1000 to 2000 and average cost falls from 40 to 32. Find the percentage saving.	Subtract, divide by the original, times 100	20 percent
HHI calculation	Shares are [30, 25, 20, 15, 10]. Find HHI, then HHI after the third and fourth firms merge.	Square, add, re-square after the merger	2250 and 2850
Evaluation	Must this merger harm consumers?	State concentration, state scale economies, state the uncertainty	Not necessarily; it depends on whether savings reach prices

Sanity check (teach this): Sanity checks: HHI must lie between 0 and 10000 and the shares must add to 100; a merger can only raise or hold HHI, so a fall means an arithmetic slip (2250 to 2850 here); average cost must fall for economies of scale, and rising from 32 back to 40 is the diseconomies direction; a percentage saving must be below 100, otherwise the denominator is wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
Classify three cases with one reason each: two bus operators merge, a shoe factory buys a shoe shop, a telecom firm buys a broadcaster.	Horizontal, vertical, conglomerate	Whether both sorting questions are used
Output 1000 to 2000, average cost 40 to 32. Find the saving and the change in total cost.	20 percent; total cost rises from 40000 to 64000	Whether average and total cost are separated
Shares [30, 25, 20, 15, 10] with the third and fourth merging. Find HHI before and after.	2250 and 2850	Whether shares are re-squared

06 Common mistakes and what to say

Mistake	Student writes	Say this
Calling a fall in total cost economies of scale	Total cost is lower after the merger, so there are economies of scale.	Economies of scale mean lower average cost; total cost usually rises after a merger.
Swapping horizontal and vertical	A bakery buying a flour mill is horizontal because both are food.	Same industry is not the same stage; upstream buying downstream is vertical.
Adding shares for HHI	The shares add to 100, so HHI is 100.	HHI is a sum of squares; adding shares badly understates the weight of large firms.
Assuming mergers always harm consumers	A merger raises price, so it always harms consumers.	Check whether scale economies cut costs and whether prices actually rose; the answer must be conditional.

07 Homework (paper, section and question numbers)

Redo this chapter — Redo practice items 1 to 3; item 3 must show both sums of squares
Economics chapter index — Open the market structure chapter and do item 2, comparing oligopoly and monopoly pricing
Case card — Find one Hong Kong merger story and write its type, two motives and one consumer effect in one line each

08 Exit check (last 5 minutes)

Question	Answer	Decision
Horizontal versus vertical	Same stage versus upstream and downstream	If not, reteach Act 1
Do economies of scale concern total or average cost	Average cost	If total, reteach Act 2
HHI for shares [30, 25, 20, 15, 10]	2250	If 100, reteach Act 3

09 Tutor FAQ

Must HHI thresholds be memorised?

DSE only needs the direction and comparison, not the threshold numbers.

Does a merger always create economies of scale?

No, it depends on integration costs and management capacity, so read the data given.

What is the point of a conglomerate?

Risk spreading and shared finance or brand, with no direct cost saving in the core business.

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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.