Question 279698
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 90 minutes. How quickly can all three fill the pool together? (Answer is 15 minutes - I have no clue how to even start!
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Rates at which each can fill the pool:
Jim: 1 pool per 30 mins
Sue: 1 pool per 45 mins
Tony: 1 pool per 90 mins
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Let x = minutes all three can fill the pool
then 
x(1/30 + 1/45 + 1/90) = 1
The 1 on the right represents 1 pool
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Multiply both sides by 90:
x(3 + 2 + 1) = 90
6x = 90
x= 90/6
x = 15 minutes