help me solve this question from percentage chapter-mathematics?

Question

The price of raw materials has gone up by 15%,labour cost has also increased by 25% of the cost of raw material . By how much "%" should there be a reduction in the usage of raw materials so as to keep the cost same..???? only one answer is correct from the following:17%,24%,28%..Correct answer is 17%...HOW??????

Answer 1

I can almost justify 28%, however 17% makes no sense to me.

Sorry man