One pipe can fill a pool in 9 hours. Another pipe can fill the pool in 6 hours. How long would it take them to fill the pool if they were working together?

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

So in 4 hours, the pool will be filled.