The consumer buys every unit whose willingness to pay is at least the market price, then Consumer's Surplus is total willingness to pay minus actual payment.

The consumer buys a unit only so long as his willingness to pay for it is at least equal to the market price of Rs. 4. Checking each unit: unit 1 (Rs. 10 ≥ 4, buy), unit 2 (Rs. 8 ≥ 4, buy), unit 3 (Rs. 6 ≥ 4, buy), unit 4 (Rs. 4 ≥ 4, buy — exactly at the margin), unit 5 (Rs. 2 < 4, do NOT buy, since it would cost him more than it is worth to him). So he buys 4 units.

Total willingness to pay for these 4 units: $10+8+6+4=28$. Actual amount paid, at Rs. 4 per unit for 4 units: $4\times4=16$.

$\text{Consumer's Surplus} = 28-16=12$ …

Including the 5th unit in the calculation, even though its willingness to pay (Rs. 2) is below the market price, is a common error — a rational consumer would never buy a unit that costs more than it is worth to him. Another mistake is reporting the …