A cistern flows into 4 cubic meters of water per minute. If 5 taps are turned on, the water in the pool can be drained in 2.5 hours. If 8 taps are turned on, the water in the pool can be drained in 1.5 hours. Now turn on 13 faucets and ask how many hours it will take to drain the water in the pool.
一个蓄水池，每分钟流入4立方水，如果打开5个水龙头，2.5小时就可把水池里的水放完，如果打开8个水龙头，1.5小时就可把水池里的水放完．现在打开13个水龙头，问多少小时可把水池的水放完．
<gadget id="calculator">2.5 * 5</gadget>
<output>12.5</output>

<gadget id="calculator">1.5 * 8</gadget>
<output>12</output>

<gadget id="calculator">12.5 - 12</gadget>
<output>0.5</output>

<gadget id="calculator">2.5 - 1.5</gadget>
<output>1</output>

<gadget id="calculator">0.5 / 1</gadget>
<output>0.5</output>

<gadget id="calculator">0.5 * 2.5</gadget>
<output>1.25</output>

<gadget id="calculator">12.5 - 1.25</gadget>
<output>11.25</output>

<gadget id="calculator">13 - 0.5</gadget>
<output>12.5</output>

<gadget id="calculator">11.25 / 12.5</gadget>
<output>0.9</output>

<result>0.9</result>