There are 9 water pipes in a reservoir, one of which is the water inlet pipe, and the other 8 are the same outlet pipes. It is known that there is a certain volume of water in the reservoir, and the water inlet pipe is filling the reservoir at a uniform speed. If all 8 outlet pipes are opened, it will take 3 hours to drain all the water in the pool; if 5 outlet pipes are opened It takes 6 hours to drain all the water in the pool. If all the water in the pool is drained within 9 hours, how many outlet pipes need to be opened at the same time.
有一个蓄水池装有9根水管，其中一根为进水管，其余8根是相同的出水管．已知储水池内有一定体积的水，并且进水管正以均匀的速度向这个蓄水池注水，如果8根出水管全部打开，需要3小时把池内的水全部排光；如果打开5根出水管，需要6小时把池内的水全部排光．如果在9小时内把水池中的水全部排光，需要同时打开多少根出水管．
<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">30 - 24</gadget>
<output>6</output>

<gadget id="calculator">6 - 3</gadget>
<output>3</output>

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

<gadget id="calculator">2 * 6</gadget>
<output>12</output>

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

<gadget id="calculator">2 * 9</gadget>
<output>18</output>

<gadget id="calculator">18 + 18</gadget>
<output>36</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>