First, convert 3/5 to a percent by multiplying it by 20/20, getting 60/100, or 60%. Then, find 60% of 40 by multiplying 60 by 40 to get 2,400 and divide it by 100 to get 24, meaning Mark spend $24 on both books. Each of the books cost $12, as 24/2 is 12.
