I used the given information to create a ratio (1 driveway per 4 boys and 2 hours) and then multiplied that by the given number of boys. Then I rearragned to find time per driveway.

Or one could use the more familiar rate*time = output

It's useful to be familiar with the many different ways to solve work/output questions.

Take, for example, this question:

If 3 identical machines can make a total of 16.1 kilograms of cookie dough in 5.4 minutes, how many kilograms of cookie dough can 6 machines make in 10.8 minutes?
A) 16.1
B) 32.2
C) 48.3
D) 64.4
E) 96.6

Answer:

As you can see, some of the above strategies will be better (i.e., faster) than others.

Cheers,
Brent

I agree.

(1 drive way / 2 hours) / 4 boys
(.25 drive way / 2 hours) / boy

(1.25 drive way / 2 hours) / 5 boys.

120 minutes / 1.25 drive ways = 120 / 5/4 = 120 * 4/5 = 480 / 5 = 96

So 95 minutes.

I have no clue why you can't do a proportion. 4 boys take 2 hours, so how long can 5 boys do it..
4 /2 = 5 / x. x=2.5 hours, so 150 minutes. The question is wrong or worded terribly. Wouldn't be on the GRE at all.

Intuitively, your answer doesn't make sense. Your answer is saying that if there are more boys working on the drive way, then it will take longer. Your total time, 150 minutes, is longer than the original time with 4 boys, 120 minutes

An equality requires that the ratio between the numerator and denominator stay constant. That's not the relationship we have when more people are working on a driveway. More people implies a shorter amount of time so a ratio does not make sense here.

If you want a formula for solving problems like this use the "work formula". $r * t * n = w$ where r = rate, t = time, n = number of people, w = work.
We start off with the equation $r * 120 * 4 = w$ so we solve for r. Then after solving for r we do $r * t * 5 = 1$ and solve for t.

Hope that helps.

I see now. That's a different work formula than the GRE tests. All we have to know is work = rate x time. I never saw work questions like this in my life before, so I know it's not going to be on the GRE. Thanks for the clarification anyway.

I partially disagree on what you asserted.

Even the most difficult GMAT problems at their core use the basics formula. Then they are tricky because of the wording.

So you cannot be sure 100% that a problem like this will NOT be tested on the GRE.

I think is best to learn a touch more from what we could expect on the GRE. We do not know. Could come in handy somehow.

Regards

Rate and time have a RECIPROCAL relationship.
TWICE the rate implies HALF the time.
THREE times the rate implies 1/3 of the time.



Increasing the number of workers from 4 to 5 = 5/4 of the original number of workers, implying that 4/5 of the original 120 minutes will be needed:
4/5 * 120 = 96 minutes

4boys=120mins
1boy=4*120
5boys=(4*120)/5=96mins