A 500-meter-long pipeline has been repaired (1/5), how many meters are left unrepaired.
一段500米长的管道，已经维修了(1/5)，还剩多少米没有维修．
<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">500 * (4/5)</gadget>
<output>400</output>

<result>400</result>