pool has a drainpipe that is normally open at the bottom and several inlet pipes of the same thickness are installed at the top. When 4 inlet pipes are opened, it takes 5 hours to fill the pool. When 2 inlet pipes are opened, it takes 15 hours. It takes 2 hours to fill the pool; now the pool needs to be filled within 2 hours, so at least how many inlet pipes must be opened.
一个水池，底部安有一个常开的排水管，上部安有若干个同样粗细的进水管，当打开4个进水管时，需要5小时才能注满水池，当打开2个进水管时，需要15小时才能注满水池；现在需要在2小时内将水池注满，那么至少要打开多少个进水管．
<result>9</result>