It takes 20 days for a project to be constructed by engineering team A, and 30 days for construction by engineering team B. How many days will it take for the two teams to jointly complete the project?
一项工程，由甲工程队修建要20天，由乙工程队修建要30天，两队合修需要多少天．
<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>