It takes 12 hours for A and B to complete a batch of parts together. If it takes 20 hours for A to complete it alone, how many hours will it take for B to complete it alone?
一批零件，甲乙两人共同完成需要12小时．如果由甲单独完成需要20小时，如果由乙单独完成需要多少小时．
<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>