A project, A and B cooperate to complete in 12 days, B alone 20 days to complete, how many days A alone to complete.
一项工程，甲、乙合作要12天完成，乙独做20天完成，甲独做多少天完成．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/12) - (1/20)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / (1/30)</gadget>
<output>30</output>

<result>30</result>