A project can be completed in 20 days by A alone, 30 days by B alone, how many days will A and B work together to complete?
一项工程甲独做20天完成，乙独做30天完成，甲、乙合做几天完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

<result>12</result>