If the selling price of 12 articles equals the cost price of 15 articles of the same kind, compute the gain percentage.

Introduction:
Ratio statements that compare totals at selling and cost prices allow you to infer the per-unit selling price relative to per-unit cost price. From that relation, the profit percentage follows directly.

Given Data / Assumptions:

• 12 units at selling price equal 15 units at cost price.
• All units are homogeneous.

Concept / Approach:
Let s be SP per unit and c be CP per unit. Then 12s = 15c implies s = (15/12)c = 1.25c. Profit% = (s - c)/c * 100.

Step-by-Step Solution:
From 12s = 15c, s = 1.25cProfit per unit = s - c = 0.25cProfit% = 0.25c / c * 100 = 25%

Verification / Alternative check:
Assume c = 100. Then s = 125. For 12 units, SP = 1500 and CP of 15 units is also 1500, which is consistent with the premise and confirms 25% profit per unit.

Why Other Options Are Wrong:

• 66 2/3% and 80%: far too large; misinterpret the ratio.
• 20%: arises from mixing article counts or using partial differences incorrectly.

Common Pitfalls:

• Using 12/15 directly as profit percentage or subtracting counts without converting to unit prices.

Final Answer:
25%