The height of 8 books stacked is 168 mm, and the height of how many books are stacked is 231 mm.
8本书摞起来高168毫米，多少本书摞起来高231毫米．
<gadget id="calculator">168 / 8</gadget>
<output>21</output>

<gadget id="calculator">231 / 21</gadget>
<output>11</output>

<result>11</result>