The cistern has a drainage pipe and a water inlet pipe. Opening the water inlet pipe alone can fill the whole pool in 5 hours, and opening the drain pipe alone can drain the water in 3 hours. Now fill the cistern with water and open the water injection pipe at the same time and the drainpipe, how long does it take to drain the water?
蓄水池有一根排水管和进水管，单开进水管5小时可以把全池注满，单开排水管3小时可把满池水排完，现将蓄水池装满水，同时打开注水管和排水管，需要多长时间可以将水排完？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) - (1/5)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>