Problem Solving

Hi AAPL,

We're told that a pipe fills a pool in 3.5 hours and that the flow of water is changed so that the pipe's rate is HALVED. We're asked how long it would then take for the pipe to fill 2/3 of the pool. While this question might look a little 'scary', the math involved is relatively low-level arithmetic.

If you 'halve' a rate, then you essentially DOUBLE the amount of time that it takes to complete a task. Thus, if a pipe normally fills an entire pool in 3.5 hours - then you HALVE that rate - it will then take 7 hours to fill that same pool. We're asked to fill 2/3 of the pool, so it will take 2/3 of 7 hours = (2/3)(7) = 14/3 = 4 2/3 hours.

Final Answer: A

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

AAPL wrote:A pipe fills a pool in 3.5 hours. If the flow of water is changed so that the pipe's rate is halved, then how long will it take for the pipe to fill 2/3 of the pool?

A. 4 hours and 40 minutes
B. 3 hours and 10 minutes
C. 2 hours and 20 minutes
D. 7 hours
E. 2/21 hours

The initial rate for the pool is 1/3.5, so when the rate is halved the new rate is 1/7.

So it will take (2/3)/(1/7) = 14/3 = 4 2/3 hours = 4 hours and 40 minutes.

Answer: A

It's often easy to do any question of this type by using real numbers.

For this question, imagine that it takes 21 gallons to fill the pool. (I specifically chose a multiple of 3.5 and 3 to make the math simple.) So Pool = 21 gallons.

Next, break down each person/machine/pipe rate into a number of gallons per hour and from there, you can usually figure it out.

In this case, filling the pool in 3.5 hours means that the pipe fills at a rate of 6 gallons per hour.
So half that rate would be 3 gallons per hour.
So how long would it take to fill 2/3 of the pool? 2/3 of 21 = 14 gallons.
14 gallons at a rate of 3 gallons per hour means time would be 14/3 or 4 and 2/3 hours.