Confusion with rates and work

Question

I've been trying to solve this problem :

Audrey 4 hours to complete a certain job. Ferris can do the same job in 3 hours. 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 ?

My attempt was to consider the duration of Ferris' each individual break as 'b' hours. Hence, 3 breaks would contribute to 3b hours. Now, since this is the additional time that Ferris takes, therefore the rate of his work drops from 1/3 jobs/hour to 1/(3 + 3b) jobs/hour.

Hence, their combined rate would be (1/4) + 1/(3 + 3b) jobs/hour. Using this I'm arriving at a result of b = 20 minutes, which is wrong. Could anyone please explain why this approach is wrong?

Answer 1

Now, since this is the additional time that Ferris takes, therefore the rate of his work drops from 1/3 jobs/hour to 1/(3 + 3b) jobs/hour

This is wrong implication.

You can solve this problem simply by:

1. Computing part of work done by Audrey using time spent on whole process and his work ratio, then rest of work had to be done by Ferris.
2. Computing how long Ferris actually worked by his original work ratio and part of the job he made.
3. Time spent on leisure is difference between time spent on whole process and time spent by Ferris on work.