Bookstore promotion, 10 yuan a story book series, buy four get one free, Xiao Ming bought a set of five, how much is each book cheaper on average.
书店促销，10元一本的故事书系列，买四赠一，小明买了一套五本，平均每本便宜多少元．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">10 * (4/5)</gadget>
<output>8</output>

<gadget id="calculator">10 - 8</gadget>
<output>2</output>

<result>2</result>