Question 538098
Hose can fill the pool in 21 hours, so it does 1/21 of the job per hr.
Drain can empty the pool in 24 hours, so it does 1/24 of the job per hr.
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(1/21 - 1/24)*x hr = 1 whole job
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((24-21)/(21*24))*x = 1
(3/504)*x = 1
3x = 504
x = 168
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Answer:  With the hose filling the pool and the drain emptying the pool simultaneously, it will take 168 hr to fill the pool.
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Check the answer
168/21 - 168/24 = ?
8 - 7 = 1
Correct.
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Done.