There are two water pipes in a pool. It takes 20 minutes to open pipe A to fill the pool, and pipe B to fill the pool in 30 minutes. How long does it take for both pipes to fill the empty pool?
一个水池有两个水管，单开甲管20分注满水池，单开乙管30分可将一池水放完，两管齐开几分钟注满空池？
<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>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<result>60</result>