If the cost-price of 18 articles is equal to the selling price of 15…

The correct answer is Option (1) → 20%

Step 1: Let the cost price (C.P.) of 1 article = ₹1

• Then C.P. of 18 articles = 18 × 1 = ₹18
• Selling price (S.P.) of 15 articles = C.P. of 18 articles = ₹18

$\text{S.P. of 1 article} = \frac{18}{15} = 1.2$

Step 2: Gain per article

$\text{Gain} = \text{S.P.} - \text{C.P.} = 1.2 - 1 = 0.2$

Step 3: Gain percentage

$\text{Gain }\% = \frac{\text{Gain}}{\text{C.P.}} \times 100 = \frac{0.2}{1} \times 100 = 20\%$