Two engineering teams dig a tunnel. Team A alone needs 20 days to complete it, and team B needs 30 days to complete it alone. How many days will it take for two teams to mine together?
两个工程队挖一条隧道，甲队单独完成需要20天，乙队单独完成需要30天．如果两队合挖需要几天完成？
<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>