<PRE><B>Question 99510</B>
2 pumps-
1 fills a tank in 5 hours
1 fills a tank in 7 hours--
how many hours to fill the tank with both pumps filling it at the same time?
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1st tank DATA;
Time + 5 hr/job ; Rate = 1/5 job/hr
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2nd tank DATA:
time = 7 hr/job ; Rate = 1/7 job/hr
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Together DATA:
time = x hr/job ; Rate = 1/x job/hr
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EQUATION:
rate + rate = together rate
1/5 + 1/7 = 1/x
Multiply thru by 35x to get:
7x + 5x = 35
12x = 35 
x = 35/12 = 3 11/12 hr = 3 hr 55 minutes (time to do the job together)
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Cheers,
Stan H.
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