It takes 20 days for A to complete a project alone, and 12 days for A and B to complete it together. If B does it alone, how many days will it take to complete?
一项工程，甲单独做需要20天完成，甲、乙合做需要12天完成，如果乙单独做需要多少天完成？
<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>