job is done by one person alone. A takes 30 minutes to complete, and B takes 20 minutes to complete. If the two cooperate, how many minutes can they complete?
一件工作，由一人单独做，甲要30分钟完成，乙要20分钟完成，如果二人合作，多少分钟可以完成．
<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>