Inflow and Outflow

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During a particular period, water enters a partially filled reservoir at a constant rate through a mountain stream. At the same time, water is pumped out of the reservoir at a constant rate through an outlet pipe. At what rate, in gallons per minute, is the amount of water in the reservoir increasing?

(1) The amount of water initially in the reservoir is 1800 gallons.

(2) Water is pumped into the reservoir at a rate of 8 gallons per minute and out of the reservoir at a rate of 20 gallons every 3 minutes.

Review: Inflow and Outflow




Explanation

We would conjecture that if you see a GMAT question that has a tank of liquid, there is about a 33% chance it's a geometry question, a 33% chance it's a solution question, a 33% chance it's a combined rate question, and about a 1% chance that it's something else. This is a combined rate question: all we have to know is the rate of water in and the rate of water out. The net rate will be the difference between those two rates. On to the data statements. Separately First. Statement (1) is irrelevant; insufficient. Statement (2) gives us the two rates we need, so it is sufficient.

The correct answer is (B).


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