JAMB 1999 · UME · Q3

A trader bought 100 oranges at 5 for ₦1.20; 20 oranges got spoilt and the remaining were sold at 4 for ₦1.50. Find the percentage gain or loss.

Worked solution (try it first)

Cost: 100 oranges are 20 lots of 5, so they cost $20 \times 1.20 =$ ₦24.
Sales: 80 good oranges are 20 lots of 4, so they sell for $20 \times 1.50 =$ ₦30.
Gain $= 30 - 24 =$ ₦6, and $\dfrac{6}{24} \times 100\% = 25\%$ gain, option B.