Question 29827
First thing you need to do is get them to equal the same thing.  Which is one job. they all added will equal to 1.
Jim can fill a pool carrying bucks of water in 30 minutes so think of it as 1/30 X (X is a unit of time). Sue can do the same job in 45 minutes which is 1/45 X. Tony can do the same job in 1 ½ hours lets change that to minutes which is 90 minutes so it would be 1/90 X. How quickly can all three fill the pool together?  here it is 1/30 X + 1/45 X + 1/90 X = 1
we take the GCD which is 90 so it is 
3/90 X + 2/90 X + 1/90 X = 1
6/90 X = 1
X = 90/6
X = 15
they do it in 15 minuts