Question 136731
jill does this in 30 min
bob does it in 45
sue takes an hour and a half
how long does it take all of them to do it together
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You have to realize you cannot add times; you have to add rates.
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Jill DATA:
Time = 30 min/job ; Rate = 1/30 job/min
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Bob DATA:
Time = 45 min/job ; Rate = 1/45 job/min
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Sue DATA:
Time = 90 min ; Rate = 1/90 job/min
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Together DATA:
Time = x min/job ; Rate = 1/x job/min
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EQUATION:
rate + rate + rate = together rate
1/30 + 1/45 + 1/90 = 1/x
Multiply thru by 90x to get:
3x + 2x + x = 90
6x = 90 
x = 15 (amount of time to do the job if they work together)
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Cheers,
Stan H.