Q.A consumer's willingness to pay for successive units of a good is Rs. 15, 12, 9, 6, and 3 for units 1 to 5 respectively. If the market price is Rs. 6 per unit, find the number of units he buys and his Consumer's Surplus.

Buy every unit whose willingness to pay is at least Rs. 6, then subtract actual payment from total willingness to pay.

Units bought: 1–4 (15, 12, 9, 6, all ≥ 6); unit 5 (Rs. 3) is not bought. Total willingness $=15+12+9+6=42$. Payment $=6\times4=24$. Surplus $=42-24=18$.

Checking each unit against the market price of Rs. 6: unit 1 (Rs. 15 ≥ 6, buy), unit 2 (Rs. 12 ≥ 6, buy), unit 3 (Rs. 9 ≥ 6, buy), unit 4 (Rs. 6 ≥ 6, buy — exactly at the margin), unit 5 (Rs. 3 < 6, do not buy). So 4 units are purchased.

Total willingness to pay for these 4 units: $15+12+9+6=42$. Actual payment at Rs. 6 per unit: $6\times4=24$.

Checked unit by unit: surplus is $15-6=9$, $12-6=6$, $9-6=3$, $6-6=0$; summing gives $9+6+3+0=18$, matching the aggregate calculation.

Forgetting that the marginal (4th) unit, bought exactly at the market price, contributes ZERO to the surplus — students sometimes wrongly add a positive surplus for every unit bought, including the marginal one.