project alone will take 20 days and B alone will take 30 days. How many days will it take for A and B to cooperate?
一项工程，甲单独完成要20天，乙单独完成要30天．甲乙合作要多少天．
<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>