There is a pool of spring water, from the bottom of which spring water gushes out continuously, and the same amount of spring water gushes out every hour. If 10 water pumps can be used to drain the water in 20 hours, and 15 similar water pumps can be used to drain the water in 10 hours, how many hours can the water be drained with 30 such water pumps?
有一池泉水，泉底不断涌出泉水，且每小时涌出的泉水一样多．如果用10台抽水机20小时可以把水抽干，用15台同样的抽水机10小时可以把水抽干，那么用30台这样的抽水机多少小时可以把水抽干．
<gadget id="calculator">20 * 10</gadget>
<output>200</output>

<gadget id="calculator">15 * 10</gadget>
<output>150</output>

<gadget id="calculator">200 - 150</gadget>
<output>50</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">50 / 10</gadget>
<output>5</output>

<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">200 - 100</gadget>
<output>100</output>

<gadget id="calculator">30 - 5</gadget>
<output>25</output>

<gadget id="calculator">100 / 25</gadget>
<output>4</output>

<result>4</result>