Let the cost of publishing the book to the publisher be X. When a commission of 10% is given on the marked price M of a book, the publisher gains 20%.

20% of X is 10% of the marked price.

0.2*X = 0.1*M

0.2 = 0.1*M/X ...(1)

When the commission...

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Let the cost of publishing the book to the publisher be X. When a commission of 10% is given on the marked price M of a book, the publisher gains 20%.

20% of X is 10% of the marked price.

0.2*X = 0.1*M

0.2 = 0.1*M/X ...(1)

When the commission is increased to 15% of the marked price, instead of 0.1, we write 0.15 in (1)

M/X = 2

The gain of the publisher is 0.15*2 = 30%

**If the commission is increased to 15%, the publisher makes a gain of 30%.**