To produce a batch of parts, it will take 12 days for A and B to do it together, and 20 days for A to do it alone. How many days will it take for B to do it alone?
生产一批零件，甲乙合做需12天完成，甲单独做20天可以完成，若乙独做需多少天完成．
<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>