Gmat Gre Work, Rate, Time

Audrey 4 hours to complete a certain job. Ferris can do the same job in 3hours. Audrey and Ferris decided to collaborate on the job, working at their respective rates. While Audrey worked continuously, Ferris took 3 breaks of equal length. If the two completed the job together in 2 hours, how many minutes long was each of Ferris’ breaks ?

A. 5
B. 10
C. 15
D. 20
E. 25




A has a rate of 1/4 (1 job in 4 hours) and F has a rate of 1/3 (1 job in 3 hours). 
LCD of 4/12 (for F) and 3/12 (for A).  

We know that A worked for the full 2 hours 
so A did 6/12 of the job

If F took no breaks, he would have completed 8/12, but we only need 6/12 so we have a difference of 2/12. 
F needs to take breaks for a total of 1/4 of the total time.
1/4 of two hours is 30 mins 
and he took 3 breaks (30/3=10). 
So each break = 10 mins 

B