Each book is 16 yuan, buy three books and get one free, buy three books at a time, how much is each book cheaper.
每本书16元，买三本送1本，一次买三本，每本便宜多少元．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">16 * (3/4)</gadget>
<output>12</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<result>4</result>