Working together, 7 identical pumps can empty a pool in 6 hours. How many hours will it take 4 pumps to empty the same pool?
A. 4 2/3
C. 9 1/3
D. 9 3/4
E. 10 1/2
7 pumps working for 6 hrs complete one work i.e. you need 7*6 = 42 pump-hours to finish that work. This means that 1 pump will take 42 hrs to complete the work or 42 pumps will take 1 hr to complete the work.
Now, you can have quickly calculate no of hrs required for any number of pumps or number of pumps required for any given number of hrs.
Given 4 pumps, it will take 42/4 = 10.5 hrs to finish the work
Given 8 pumps, it will take 42/8 = 5.25 hrs to finish the work
Given that you need to complete the work in 7 hrs, you need 42/7 = 6 pumps for it.
Given that you need to complete the work in 3 hrs, you need 42/3 = 14 pumps for it.
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