It takes 20 days for Uncle Wang to dig a canal alone, and 30 days for Uncle Li to do it alone. How many days will it take for the two to work together?
挖一条水渠，王伯伯独自完成需要20天，李叔叔独自完成需要30天，如果两人合作，需要几天才能完成？
<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>