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Bunuel
A tank of volume 432 cubic feet has one inlet pipe and2 outlet pipes. The inlet pipe fills water into the tank at the rate of 4 cubic inches/min and the 2 outlet pipes empty it out at the rates of 14 cubic inches/min and6 cubic/min respectively. If all 3 pipes opened when the tank is full, in how much time is it emptied?

(A) 777.6 hrs
(B) 90 hrs.
(C) 80 hrs
(D) 77.76 hrs.
(E) 70 hrs.
Solution:

We see that the net rate of the pipes is 4 - 14 - 6 = -16 in^3/min. That is, water is pouring out of the tank at a rate of 16 in^3/min. Since 432 ft^3 = (432 x 12^3) in^3, it takes (432 x 12^3) / 16 = (27 x 12^3) minutes or (27 x 12 x 12^2) / 60 = (27 x 12^2) / 5 = 777.6 hours to empty the tank.

Answer: A
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normally in this type of questions official GMAT gives the conversion, if not how an not american can know it :)
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