Question 504023
L = amount of water removed by the larger pump per minute
S = amount of water removed by the smaller pump per minute
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working together, they can do 1 whole job in 90 minutes
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90*(L+S) = 1
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working alone, the larger pump can do the job in 120 minutes (2 hr)
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120L = 1
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L = 1/120 of the job per minute
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substitute to find S
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90*(L+S) = 1
90*(1/120 + S ) = 1
1/120 + S = 1/90
S = 1/90 - 1/120
S = 4/360 - 3/360
S = 1/360
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So the smaller pump can do the job at a rate of 1/360 per minute.
360 minutes = 6 hr.
That means it would it it 6 hours working alone.
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Always check your work.
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90(1/120 + 1/360) = ?
90/120 + 90/360 = 3/4 + 1/4 = 1
Correct.
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Re-read the question to make sure you answer it.
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Answer:  The smaller pump would take 6 hrs to do the job working alone.
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Done.