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ECON 103

When prices change, how much do buyers react?

Connect elasticity to buyer and seller responses, revenue rectangles and the midpoint method. Learn the ideas, then solve 10 questions with worked answers.

Measuring responses · Step 1 of 12

Use the average as the percentage base

Cloudberry tokens: demand and revenue
Price ($ per token)
QAQBPBPAABD
Quantity (tokens)
Shaded area: Total revenue.D. Marked point A. Marked point B.
D: demand for tokensA: 40 tokens at $16 eachB: 60 tokens at $12 eachTotal revenue
Cloudberry tokens are an invented good, with demand P = 24 − 0.20 × Q. Price is dollars per token and Q is tokens bought per trading period. The midpoint method divides each change by the average of its two values, so reversing the journey keeps the same elasticity.
Try it: Move either point along demand. Try reversing which point has the larger quantity.
Point A: quantity (tokens)40
Point B: quantity (tokens)60
Quantity change
40.00%
Price change
−28.57%
P at A = (24 − 0.20 × (40)) = $16.
P at B = (24 − 0.20 × (60)) = $12.
Quantity: average = (40 + 60) ÷ 2 = 50.
Change = (60 − 40).
Midpoint % change = (60 − 40) ÷ [(40 + 60) ÷ 2] × 100 = 40.00%.
Price: average = (16 + 12) ÷ 2 = 14.
Change = (12 − 16).
Midpoint % change = (12 − 16) ÷ [(16 + 12) ÷ 2] × 100 = −28.57%.
Check yourself
Which base does the midpoint quantity change use?

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