For a 5-meter-long wire, first use up (1/5) and then use up (1/5) meter, how much of the wire is left.
5米长的电线，先用去(1/5)，再用去(1/5)米，还剩这根电线的多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 5</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">1 - (1/5) - (1/25)</gadget>
<output>19/25 = around 0.76</output>

<result>19/25 = around 0.76</result>