A pile of sand weighs 20 tons, if (1/5) is used up, how many tons are left?
一堆沙重20吨，用去(1/5)，还剩多少吨．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<result>16</result>