There is a cistern equipped with 9 water pipes, one of which is the water inlet pipe, and the other 8 are the same outlet pipes. The water inlet pipe starts to fill the cistern with water at a uniform speed. After some water is injected into the pool, Some people want to open the outlet pipes 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. In order to drain the water in the pool. Question: 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注水，池内注入一些水后，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个小时可将池内的水排光；若仅打开3根出水管，则需要18小时才能将池内的水排光．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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