A pile of coal is used up (1/7) every day on average, how many days it will all be used up.
一堆煤，平均每天用去(1/7)，多少天全部用完．
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / (1/7)</gadget>
<output>7</output>

<result>7</result>