Joseph is going to pay full price for the higher priced book so we start with 15 dollars. Then he pays 50% of the second book which is 10 dollars, so we find 50% of 10 and then add it to 15...
15 + 50% x 10 =
15 + .5 x 10 =
15 + 5 = 20
He pays a total of $20