How do I find the equilibrium price?

Set quantity demanded equal to quantity supplied, solve for the price, then substitute back to get the quantity.

Does a price change shift the demand curve?

No: a price change is a movement along the same curve; only non-price factors shift the whole curve.

S4 Economics Demand and Supply, Paper 1: law of demand, equilibrium and curve shifts

2026-11-01

The law of demand links price inversely to quantity demanded; equilibrium comes from Qd = Qs, and only non-price factors shift a whole curve.

Focus　Write Qd = Qs to find equilibrium, then decide whether the question is a movement along a curve or a shift of the curve.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Economics: Demand, Supply and Elasticity
Level	S4

02　Questions (10 items)

FoundationThe price of a good rises from $20 to $24 while other factors stay unchanged, and the quantity bought falls from 50 to 40. Which economic law does this show?
FoundationIn a market the demand function is Qd = 200 – 4P. How many units are demanded when the price is $25?
FoundationIn a market the supply function is Qs = 30 + 5P. How many units are supplied when the price is $10?
FoundationIn a market Qd = 120 – 3P and Qs = 30 + 3P. Find the equilibrium price and quantity.
IntermediateIf demand for a good increases (the demand curve shifts right) while supply is unchanged, what happens to the equilibrium price and quantity?
IntermediateIf supply of a good increases (the supply curve shifts right) while demand is unchanged, what happens to the equilibrium price and quantity?
IntermediateIn a market Qd = 80 – 2P and Qs = 20 + 3P. Find the equilibrium price and quantity.
ChallengeA market starts with Qd = 150 – 3P and Qs = 30 + 3P. Demand then rises by 30 units at every price while supply is unchanged. Find the old and new equilibrium price and quantity.
ChallengeIn a market Qd = 120 – 4P and Qs = 2P. The government sets a price ceiling of $15. Find the equilibrium price and the shortage under the ceiling.
Error redoA student says: “A rise in the price of a good shifts its demand curve to the right.” What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	The law of demand: with other factors unchanged, a higher price lowers the quantity demanded.	The law of demand: other things being equal, a rise in price reduces the quantity demanded.
2	100 units	Substitute P = 25: Qd = 200 – 4*25 = 100 units.
3	80 units	Substitute P = 10: Qs = 30 + 5*10 = 80 units.
4	Equilibrium price $15, equilibrium quantity 75 units.	Set Qd = Qs: 120 – 3P = 30 + 3P, so 6P = 90, P = 15 is the equilibrium price and Q = 75 the equilibrium quantity.
5	The equilibrium price rises and the equilibrium quantity increases.	A rightward shift of demand moves the equilibrium point up along the supply curve, so both price and quantity rise.
6	The equilibrium price falls and the equilibrium quantity increases.	A rightward shift of supply moves the equilibrium point down along the demand curve, so price falls while quantity rises.
7	Equilibrium price $12, equilibrium quantity 56 units.	80 – 2P = 20 + 3P gives 5P = 60, so P = 12 and Q = 80 – 24 = 56 units.
8	Old: price $20, quantity 90 units. New: price $25, quantity 105 units.	The original equilibrium is 6P = 120, so P = 20 and Q = 90. Adding 30 units of demand gives Qd = 180 – 3P, so 6P = 150, P = 25 and Q = 105.
9	Equilibrium price $20. Under the ceiling quantity demanded is 60 and quantity supplied is 30, so the shortage is 30 units.	Equilibrium is where 120 – 4P = 2P, so P = 20 and Q = 40. A ceiling of $15 is below equilibrium, giving Qd = 60 and Qs = 30, a shortage of 30 units.
10	Wrong: a price change only moves us along the same demand curve, changing the quantity demanded, not demand itself. Only non-price factors shift the whole curve.	Distinguish a movement along the curve from a shift of the curve: price is on the vertical axis, so a price change only changes the quantity demanded.

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.

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