Answer= 4 hours. The first pipe can fill one pool in nine hours. So in one hour, it can fill 1/9th of the pool. Same with the other pipe, it can fill 1/6th of the pool in other hour. So 1/9 + 1/6 would represent the first hour of filling the pool. Since 1/9 + 1/6 = .277.. , 27% of the pool is filled. Repeat this until you get to 100% (full pool).
First hour, 1/6 + 1/9 = .27 Second hour, 2/6 + 2/9 = .5 Third hour, 3/6 + 3/9 = .83 Fourth hour, 4/6 + 4/9 = 1.11
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