On The GRE you can use whatever method you want to solve a problem in the most efficient way.

Just to reach the right answer in the minor time frame.

Regards

the formula gives 0.3 of an hour and not 30 min.

How'd you know that they both completed half of the job?

In this problem it works out that way but couldn't there be a scenario where two people are working on a job and one does more work than the other?

Because

In 2 hours Audrey completed \(\frac{2}{4}=\frac{1}{2}\) of the job,

and the remaining job is done by Ferris

No, don't use this method. This is wrong .Please refer to the explanation provided by @Carcass

can you please explain in a more simpler way? I'm not able to understand it. Please !!!

It's very simple,

It says " Audrey took 4 hours to complete a certain job" but the job is completed in "2 hours"

Hence, at this rate Audrey can complete \(\frac{1}{2}\) of the work.

Next half has to be completed by Ferris and the ques says : " Ferris can do the same job in 3 hours"

meaning the remaining half of the job can be completed by Ferris in 1.5 hours or 1 hour and 30 mins or 90 mins,

BUT, he takes 120 mins (that includes the break) i.e 120 -90 = 30 mins of break , which is 10 min of each for 3 times

Audrey can complete a certain job in 4 hours, while Ferris can do the same job in 3 hours.
So, Audrey's RATE = 1/4 of the job per hour
And Ferris' RATE = 1/3 of the job per hour

Audrey and Ferris worked together on the job and completed it in 2 hours, but while Audrey worked this entire time, Ferris worked for some of the time and took 3 breaks of equal length.
Since Audrey works for the entire 2 hours, let's determine how much work she does.
At a rate of 1/4 of the job per hour, Audrey can complete 1/2 of the job in TWO hours.

This means Ferris must have completed the other 1/2 of the job

Time = output/rate
So, Ferris' work time = (1/2)/(1/3) = 3/2 hours = 90 MINUTES

So, at his normal rate of work, Ferris can complete his half of the job in 90 MINUTES, which meanshe rested for the other 30 minutes.


How many minutes long was each of Ferris's break?
Ferris took 3 breaks of equal length
If he rested for a TOTAL of 30 minutes, each break was 10 minutes long.

Answer: B

Cheers,
Brent

Why is this method wrong? I dont understand


No, don't use this method. This is wrong .Please refer to the explanation provided by Carcass

It's actually a simple question if understood and worked rightly

Assuming work to be done as 12 units (LCM of 4,3,2)

We have, Rate of Audrey 3 units/ hr. (12/4)
Rate of Ferris 4 units/hr. (12/3)

When combined even working for 2 hours we have both the rates into account but first considering Audrey

Even after working for whole 2 hours continuously without the involvement of Ferris, he would have completed only 6 units of work(3x2),

Now we got to find actual amount of time Ferris worked i.e... 6/4 =1.5 hrs.

Possibility of break here 2-1.5=0.5 hrs. =30 minutes
Three equal breaks 30/3 =10 minutes is the answer.

Thank you for this problem.