To read quantity at a stated price, start on the price axis, move across to the demand curve, then down to the quantity axis. With a table or equation, use the row or substitute the price. The skill is simple, but wrong-axis slips cost marks in every demand question.
It supports every other lesson in demand relationships, and it connects to ratio and percentage work in the percentage-base explorer and the ratio reading tool.
How do you read it step by step?
- Identify the axes. Price is vertical, quantity is horizontal, unless the diagram says otherwise.
- Locate the given price. Check units (RM per kg, RM per cup).
- Move across to the curve, then straight down. Read the quantity.
- If the data is a table, read the matching cell. If it is an equation, substitute the price.
- Write the answer with a unit: “240 kg per day”, not just “240”.
Worked example
A fictional vegetable stall at Pasar Tani Bukit Melur sells kuih. The daily demand is Qd = 480 − 80P, with P in RM per box and Qd in boxes.
| Price (RM) | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Qd (boxes per day) | 400 | 320 | 240 | 160 | 80 |
a) Quantity at RM3. The table gives 240 boxes. Check with the equation: 480 − 80 × 3 = 480 − 240 = 240.
b) Quantity at RM3.50. Assume a straight line. Halfway between 240 (RM3) and 160 (RM4) is 200. Check: 480 − 80 × 3.5 = 480 − 280 = 200 boxes.
c) Price at which 280 boxes are demanded. Read backwards: 480 − 80P = 280, so 80P = 200 and P = 2.5. The price is RM2.50. Check against the table: RM2 gives 320 and RM3 gives 240, so 280 lies between them, which fits.
d) Percentage change in quantity if price rises from RM3 to RM4. Change is 160 − 240 = −80. The base is the starting quantity, 240. So −80 ÷ 240 = −0.333, or about −33.3%.
The mistake to watch for
Mistaken answer: “At RM3.50 the quantity is 3.5 boxes” or “At RM3 the quantity is 3.”
The student read the price as the quantity, or copied the stated number onto the wrong axis. Correction: label the axes on the diagram before you read, and say aloud “price in, quantity out”.
Another slip is using the new quantity as the base for percentage change. The base is always the starting value, here 240.
Check yourself
Use the kuih schedule above.
1. What is quantity demanded at RM2?
Show answer
320 boxes per day. Check: 480 − 80 × 2 = 320.
2. What is quantity demanded at RM4.50, assuming a straight line?
Show answer
Halfway between 160 (RM4) and 80 (RM5) is 120 boxes. Check: 480 − 80 × 4.5 = 480 − 360 = 120.
3. At what price does quantity demanded become zero?
Show answer
Set 480 − 80P = 0, so 80P = 480 and P = RM6. At RM6 no boxes are demanded.
Where this leads next
Next, compare two demand curves with identical scales. To practise on mixed questions, use the demand relationships practice set.
If you read the right number but then lose marks explaining it, our online one-to-one Economics tuition can work on that step with you.