The C.P of 15 books is equal to the S.P of 18 books. Find his gain% or loss%?

Key idea: relate total cost and total selling price to find loss percent per book.

1. Let the cost price of one book be x.

2. Given: Cost price of 15 books = Selling price of 18 books, so 15x = total SP of 18 books.

3. Therefore selling price of one book = (15x) / 18 = (5/6)x.

4. Loss per book = cost − selling price = x − (5/6)x = (1/6)x.

5. Loss% = (loss / cost) × 100 = (1/6) × 100 = 100/6 = 16 2/3%.

Answer: 16 2/3% loss