gmatquant25 wrote:

Pipe A can fill a tank in 3 hours. Pipe B can empty the same tank in 2 hours. If the tank is initially two thirds full then in how many hours will the tank be completely empty in if both the pipes are turned on simultaneously?

(A)2 hours

(B)3 hours

(C)4 hours

(D)5 hours

Lets say tank contains 6 liters

Pipe A works at

\frac{6}{3}= 2 liters every hour (added)=

+2Pibe B works ar

\frac{6}{2}= 3 liters every hour (subtracted)=

-3The tank is 2/3 full so contains 4 liters, and the pipes combined give us a

-3+2=-1 liter every hour.(minus one liter every hour)

So they would take

4 hours to empty it.

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