Question 213462
You can start this problem by finding the rate (how much can be done in one minute) for each of the participants.
If Sue can fill the pool in 30 minutes, then she can fill {{{1/30}}} of the pool in 1 minute.
If Paul can fill the pool in 45 minutes, then he can fill {{{1/45}}} of the pool in 1 minute.
If Bob can fill the pool in 90 minutes, then he can fill {{{1/90}}} of the pool in 1 minutes.
So in 1 minute, working together, they can fill:
{{{1/30 + 1/45 + 1/90 = 6/90}}}={{{1/15}}} of the pool.
If they can fill {{{1/15}}} of the pool in 1 minute, then it will take them 15 minutes to fill the entire pool.