Question 186956
Think about find the rate at which each person can fill the pool.

If John can fill the pool in 30 minutes, then he can fill {{{1/30}}} in 1 minute.
If Sue can fill the pool in 45 minutes, then she can fill {{{1/45}}} of the pool in 1 minute.
If Sam can fill the pool in 90 minutes, then he can fill {{{1/90}}} of the pool in 1 minute. 
So if they work together, then all three of them can fill {{{1/30 + 1/45 + 1/90}}} of the pool in 1 minute.
Adding these fractions, you find that the three of them can fill {{{1/15}}} of the pool in 1 minute.
Well, if they can fill {{{1/15}}} of the pool in 1 minute, then it will take them 15 minutes to fill the entire pool working together.