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 does it take for the two teams to work together? (Only columns or equations do not calculate)
一项工程，由甲工程队修建，需要20天，由乙工程队修建，需要30天．两队合修需要多少天？(只列式或方程不计算)
<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>