Correct option is (b) \(13\frac 13 \)%
From the given data, the profit is 20% when the discount is 10% .
Let the profit be x% when the discount is 15%.
Now, we know that, the profit is inversely proportional to the discount.
∴ x : 20 = 10 : 15
⇒ \(\frac x{20} = \frac{10}{15}\)
⇒ \(x = \frac{10}{15} \times 20\)
⇒ \(x = 13 \frac 13\)
So the gain = \(13 \frac 13\)%.