During a sale at a bookstore, joseph buys a book at full price. he is given a 50 percent discount on a second book of equal or less value. if joseph buys two books that have a full price of $15 and $10, by what percent is the total cost of the two books reduced during the sale? (a) 5% (b) 20% (c) 25% (d) 30% (e) 50%