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?