bundle of iron wire is used up (3/5) for the first time, and (7/20) for the second time, how much is left useless?
一捆铁丝，第一次用去(3/5)，第二次用去(7/20)，还剩下几分之几没有用？
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">7 / 20</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">1 - (3/5) - (7/20)</gadget>
<output>1/20 = around 0.05</output>

<result>1/20 = around 0.05</result>