By selling 100 bananas, a fruit seller gains an amount equal to the selling price of 20 bananas. Compute his gain percentage.

Introduction:
Stated profits in terms of selling price of some quantity can be translated into algebraic relations. Once the total selling price and profit are linked, the cost price follows, enabling the profit percentage calculation.

Given Data / Assumptions:

• Let s be SP per banana.
• Total quantity = 100 bananas.
• Profit amount = SP of 20 bananas = 20s.

Concept / Approach:
Total SP for 100 bananas = 100s. With profit = 20s, total cost must be 100s - 20s = 80s. Profit percentage = profit / cost * 100.

Step-by-Step Solution:
SP_total = 100sProfit = 20sCost = 100s - 20s = 80sProfit% = 20s / 80s * 100 = 25%

Verification / Alternative check:
Take s = Rs 5. Then SP_total = 500, profit = 100, cost = 400, giving 100/400 * 100 = 25% gain.

Why Other Options Are Wrong:

• 10% and 15%: underestimates from incorrect cost base.
• 20%: still too low; profit should be compared to cost, not to SP.

Common Pitfalls:

• Treating the 20 bananas as 20% of selling price and assuming that equals the percentage gain directly.

Final Answer:
25%