Working together, two pumps can drain a certain pool in 4 hours. If it takes the older pump 9 hours to drain the pool by itself, how long will it take the newer pump to drain the pool on its own?

Working together, two pumps can drain a certain pool in 5 hours.

If it takes the older pump 15 hours to drain the pool by itself,

how long will it take the newer pump to drain the pool on its own?

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you can do this in your head but here is an algebra equation way

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Let t = time required by new pump to drain the pool

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Let the completed job (drained pool) = 1

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+ = 1

Multiply by 15t

t (5) + 15 (5) = 15

5t + 75 = 15t

75 = 15t - 5t

75 = 10t

t = 75/10

t = 7.5 hrs new pump alone

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Check this

5/15 + 5/7.5 =

.33 +.67 = 1

Eight hours because it takes four hours to drain the pool with two new pumps so you multiply 4 x 2 which is 8.