pool has an inlet pipe that continuously feeds water, and there are several identical suction pipes. If 24 water pipes are used to pump water, the water in the pool can be drained in 6 hours; if 21 water pipes are used to pump water, the water in the pool can be drained in 8 hours. If 16 pumping pipes are used, how many hours can the water in the pool be drained.
一水池有一根进水管不间断地进水，另有若干根相同的抽水管．若用24根抽水管抽水，6小时即可把池中的水抽干；若用21根抽水管抽水，8小时可将池中的水抽干．若用16根抽水管，多少小时可将水池中的水抽干．
<gadget id="calculator">21 * 8</gadget>
<output>168</output>

<gadget id="calculator">24 * 6</gadget>
<output>144</output>

<gadget id="calculator">168 - 144</gadget>
<output>24</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 * 8</gadget>
<output>96</output>

<gadget id="calculator">168 - 96</gadget>
<output>72</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<result>18</result>