Working together, 7 identical pumps can empty a pool in 4 hours. How many hours will it take 5 pumps to empty the same pool?  Pick one:
A) 4 2/3
B) 9 1/4
C) 9 1/3
D) 9 3/4
E) 5 3/5
The answer is E   7 pumps take 4 hours. 1 Pump will take 4*7 hours . Hence 5 pumps will take (4*7)/5 = 5 6/10 = 5 3/5 hours
<result>E</result>
Working together, 7 identical pumps can empty a pool in 4 hours. How many hours will it take 5 pumps to empty the same pool?