Question

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?

Answer 1

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?
:
you can do this in your head but here is an algebra equation way
:
Let t = time required by new pump to drain the pool
:
Let the completed job (drained pool) = 1
:
+ = 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
;
:
Check this
5/15 + 5/7.5 =
.33 + .67 = 1

Answer 2

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