If a commission at the rate of 10% is given to a bookseller on the marked price of a book by the publisher, the publisher gains 20% . If the commission is increased to 15% , then the gain percent would be = ?

Answer: Option B

Let CP = 100 and Marked Price = x
SP = 100 + 20% of 100 = 120
10% commision was given on SP
SP = x - 10% of x
$$\begin{array}{rl}& 120=x-\frac{10x}{100}\\ & 120=\frac{9x}{10}\\ & 9x=1200\\ & x=\frac{1200}{9}=133.33\\ & \text{MP}=133.33\end{array}$$
If 15% commission was given, then
SP = 133.33 - 15% of 133.33
SP = 133.33 - 20
SP = 113.33
Gain = 113.33 - 100
% Gain = 13.33% or $$13\frac{1}{3}\%$$