Buy 3 notebooks, pay 20 yuan, get back 8 yuan, how much is each notebook.
买3本笔记本，付出20元，找回8元，每本多少元．
<gadget id="calculator">20 - 8</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<result>4</result>