A pool has a drain pipe that is normally open at the bottom, and several identical drain pipes are installed on it. When 4 inlet pipes are opened, it takes 5 hours to be filled with water. When 2 inlet pipes are opened, it takes 15 hours. Only then can the pool be filled, and now it needs to fill the pool with water within 3 hours, as long as how many water inlet pipes need to be opened?
一个水池，底部按有一个常开的排水管，上面装有若干个一样的排水管，当打开4个进水管时，需要5小时才能注满水，当打开2个进水管时，需要15小时才能注满水池，现在需要3小时内将水池注满水，只要要开几个进水管？
<result>9</result>