Question 189509
Jim can fill a pool carrying buckets of water in 30 minutes. Sue can do the same job in 45 minutes. Tony can do the same job in 1 ½ hours. How quickly can all three fill the pool together? 
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Jim does 1/30 of a pool per minute
Sue does 1/45
Tony does 1/90
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Per minute, it's 1/30 + 1/45 + 1/90 = 6/90 = 1/15 pool per minute.
15 minutes