Xiaofang goes to the bookstore to buy books, each book is 15 yuan, buy 4 books and get 1 free, how much is each book cheaper?
小芳去书店买书，每本15元，买4本送1本，每本便宜多少元？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<result>3</result>