pool is equipped with a water inlet pipe and a drain pipe. If the water inlet pipe is opened alone, the empty pool can be filled with water in 20 hours. If the drain pipe is opened alone, the water in the full pool can be drained in 15 hours. If two water pipes are opened at the same time, how many hours will it take to drain a pool full of water?
一个水池装有一个进水管、排水管，若单开进水管，20小时可将空池注满水，若单开排水管，15小时可将满池的水排完．现将两个水管同时打开，多少小时可将一满池的水排完？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

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

<result>60</result>