There is a project to be completed independently, A takes 30 days, B takes 20 days, A and B do it at the same time, how many days will it take to complete.
单独完成有一项工程，甲要30天，乙要20天，甲乙同时做，需用多少天可以完成．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

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

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

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

<result>12</result>