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 $7.50 more than Alana and Petra combined. How much does
one book cost?