SOLUTION: A water tank can be emptied by using one pump for 4 hours. A second, smaller pump can empty the tank in 7 hours. If the larger pump is started at 1:00 P.M., how much time should pa

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Question 766681: A water tank can be emptied by using one pump for 4 hours. A second, smaller pump can empty the tank in 7 hours. If the larger pump is started at 1:00 P.M., how much time should pass until the smaller pump is started so that the tank will be emptied at 4:00 P.M.?
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
A water tank can be emptied by using one pump for 4 hours. A second, smaller pump can empty the tank in 7 hours. If the larger pump is started at 1:00 P.M., how much time should pass until the smaller pump is started so that the tank will be emptied at 4:00 P.M.?
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1st pump rate: 1/4 job/hr
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2nd pump rate: 1/7 job/hr
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Work + work = job done
(1/4)3 + (1/7)x = 1
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(3/4) + (x/7) = 1
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21 + 4x = 28
4x = 7
x = 7/4 hrs
x = 1 3/4 hrs (amount of time to use the smaller pump)
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Time to start the smaller pump is 4pm - 1 3/4 hrs = 2:15 pm
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Cheers,
Stan H.