There is a cistern with 9 water pipes, one of which is the inlet pipe, and the other 8 are the same outlet pipes. At the beginning, the water inlet pipe keeps storing water to this reservoir at a uniform speed. After some water was injected into the pool, someone wanted to open the outlet pipe to drain all the water in the pool. If all 8 outlet pipes are opened, it will take 3 hours to drain the water in the pool; if only 3 outlet pipes are opened, it will take 18 hours. Q If you want to drain all the water in the pool within 8 hours, how many outlet pipes should you open at least?
有一个蓄水池装有9根水管，其中1根为进水管，其余8根为相同的出水管．开始进水管以均匀的速度不停地向这个蓄水池蓄水．池内注入了一些水后，有人想把出水管也打开，使池内的水再全部排光．如果把8根出水管全部打开，需要3小时可将池内的水排光；而若仅打开3根出水管，则需要18小时．问如果想要在8小时内将池中的水全部排光，最少要打开几根出水管？
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