Alana and Petra go to a used book sale, where every book is the same price. Alana buys 5 books and Petra buys 4 books. They run into their friend Angus,who really loves to read, and discover that he has bought 19 books. Angus received a $10 discount, but he still spent $5 more than Alana and Petra combined. How much does one book cost?
