For a water pipe, cut off the full length (3/20) for the first time, cut off the rest (3/10) for the second time, and cut off the total length of the two times.
一根水管，第一次截去全长的(3/20)，第二次截去余下的(3/10)，两次共截去全长的多少．
<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 - (3/20)</gadget>
<output>17/20 = around 0.85</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(17/20) * (3/10)</gadget>
<output>51/200 = around 0.255</output>

<gadget id="calculator">(3/20) + (51/200)</gadget>
<output>81/200 = around 0.405</output>

<result>81/200 = around 0.405</result>