Alana and Petra go to a used book sale, where every book is the same price. Alana buys 4
 books and Petra buys 2
 books. They run into their friend Angus, who really loves to read, and discover that he has bought 16
 books. Angus received a $15
 discount, but he still spent $5
 more than Alana and Petra combined. How much does one book cost?