Hard to believe this is college level but here we go! You're finding what the price is of a book that is $39.99 at a 40% discount. In order to find the answer you multiply your price times the discount in decimal form. Then subtract this value from the original.
40% ⇒ .40 (Note: Percentages are out of 100)
[39.99 x .40] = 15.996
[39.99 - 15.996] = 23.994
The bookstore pays $23.99 per copy.
