Water & Pipes

Hi All,

We're told that water is pumped into a partially filled tank at a constant rate through an inlet pipe and at the same time, water is pumped out of the tank at a constant rate through an outlet pipe. We're asked for the rate, in gallons per minute, that the amount of water in the tank is INCREASING. This is an example of a 'concept question' (meaning that you don't have to do much - if any - math to answer the question that is asked IF you recognize the concepts involved).

1) The amount of water initially in the tank is 200 gallons.

Fact 1 tells us nothing about how fast water is entering or exiting the tank, so there's no way to determine the exact rate the amount of water is increasing.
Fact 1 is INSUFFICIENT

2) Water is pumped into the tank at a rate of 10 gallons per minute and out of the tank at a rate of 10 gallons every 2.5 minutes.

With the information in Fact 2, we know how quickly water is entering the tank (10 gallons/minute) and we can calculate how quickly water is exiting the tank (10/2.5 = 4 gallons/minute, although we don't technically have to do that math to KNOW that we COULD determine that rate). Thus, the number of gallons per minute increase is 10 - 4 = 6 gallons/minute.
Fact 2 is SUFFICIENT

Final Answer: B

GMAT assassins aren't born, they're made,
Rich

smclean23 wrote:Water is pumped into a partially filled tank at a constant rate through an inlet pipe. At the same time, water is pumped out of the tank at a constant rate through an outlet pipe. At what rate, in gallons per minute, is the amount of water in the tank increasing?

(1) The amount of water initially in the tank is 200 gallons.
(2) Water is pumped into the tank at a rate of 10 gallons per minute and out of the tank at a rate of 10 gallons every 2.5 minutes.

We are given that water is flowing into a tank through an inlet pipe and out of the tank through an outlet pipe. We need to determine at what rate the amount of water in the tank is increasing.

Statement One Alone:

The amount of water initially in the tank is 200 gallons.

Knowing the initial amount of water in the tank is not enough information to determine at what rate the amount of water in the tank is increasing. Statement one alone is not sufficient to answer the question.

Statement Two Alone:

Water is pumped into the tank at a rate of 10 gallons per minute and out of the tank at a rate of 10 gallons every 2.5 minutes.

From statement two, we know the rate at which the water is flowing into the tank and also the rate at which the water is flowing out of the tank. Thus we have:

rate in = 10/1 = 10 gallons per minute

rate out = 10/2.5 = 4 gallons per minute

Thus, the amount of water is increasing in the tank at a rate of 10 – 4 = 6 gallons per minute.

Statement two alone is sufficient to answer the question.

Answer: B