A pool is equipped with a water inlet pipe and a drain pipe. If the water inlet pipe is only opened, the empty pool can be filled with water in 30 hours. If the drain pipe is opened alone, the water in the full pool can be drained in 20 hours. Now there is a pool full of water, if both pipes are open, how many hours can the full pool be drained?
一个水池装有一个进水管和一个排水管，若单开进水管，30小时可将空池注满水，若单开排水管，20小时可将满池的水排完．现在有一满池水，双管齐开，多少小时可将一满池的水排完？
<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>