Bunuel wrote:

A 500-gallon cistern is filled to half its capacity with water. At what rate, in gallons per minute, is the amount of water in the cistern increasing if both the inlet pipe and outlet pipe are open?

(1) The inlet pipe pumps water into the cistern at a rate of 7 gallons per 30 seconds.

(2) The outlet pipe pumps water out of the cistern at the rate of 12 gallons every 1 minute 30 seconds.

Since the Q. asks for the rate of water increase- we must know the inlet and outlet rates

1. Provides the inlet rate- nothing can be said whether the amount of water in the cistern would increase or decrease untill we know about the outlet rate

2. Provides the outlet rate- nothing can be said whether the amount of water in the cistern would increase or decrease untill we know about the inlet rate

Combining both would give us the answer(till now we do not know whether the increase would be positive or negative, but it is sure that we will get the answer)

{For those who still seek the answer,

Inlet rate- 14 gallons/min.

Outlet rate - 12/1.5 (1.5 is 3/2)

- 12*2/3 = 8 gallons/min.

Diff. - 6 gallons/min is the rate of increase}

Ans. C

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