There is a reservoir equipped with 10 water pipes, one of which is the water inlet pipe, and the other 9 are the same outlet pipes. The water inlet pipes are continuously filled with water at a uniform speed. If all 9 outlet pipes are opened, it will take 2 hours to drain the water in the pool. If only 5 outlet pipes are opened, it will take 6 hours to drain the water in the pool. If you want to drain the water in 4 hours, At least how many outlet pipes need to be opened at the same time?
有一蓄水池装有10根水管，其中一根为进水管，其余9根为相同的出水管，进水管以均匀的速度不停的注水，到一定水位时，有人想打开出水管，使池内的水全部排外，如果9根出水管全部打开，需2小时把池内的水排光，如果只打开5根出水管，需6小时把池内的水排光，若想4小时把水排光，至少需同时打开几根出水管？
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">30 - 18</gadget>
<output>12</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">18 + 6</gadget>
<output>24</output>

<gadget id="calculator">24 / 4</gadget>
<output>6</output>

<result>6</result>