For a 6-meter-long wire, use up (1/5) first, and then use up (1/5) meter, leaving ((())/(())) of this wire.
6米长的电线，先用去(1/5)，再用去(1/5)米，还剩这根电线的((())/(()))．
<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">6 * (4/5)</gadget>
<output>24/5 = around 4.8</output>

<gadget id="calculator">(24/5) - (1/5)</gadget>
<output>23/5 = around 4.6</output>

<gadget id="calculator">(23/5) / 6</gadget>
<output>23/30 = around 0.766667</output>

<result>23/30 = around 0.766667</result>