If a commission of 10% is given to the customer on the catalog price of a book, the publisher gains 20%. If the commission is increased to 15%, the gain is:

Let CP = 100 and Marked Price = X

SP = 100 + 20% of 100 = 120

10% commission was given on SP

SP = X – 10% of X

120 = X – 10X/100

120 = 9X/10

9X = 1200

X = 1200/9 = 133.33

MP = 133.33

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%