Here's my reasoning: Janet works at the rate (1 job)/(3 hr), and works one hour before Gary steps in to help. This means that Janet accomplishes 1/3 of the entire job by herself, leaving 2/3 of the job for the two of them to complete.
Thus, I write:
2/3 job
--------------------------- = time required for the 2 to finish the job
1 job 1 job
------- + ---------
3 hr 2 hr
The LCD here is 3(2) = 6. Mult. (2/3 job) by 6 results in 4 job, and
mult (1 job)/(3 hr) by 6 results in 2 job/hr, and mult (1 job)/(2 hr) by 6 results in 3 job /hr:
4 job 4 job
--------------------------- = --------------- = (4/5) hr (answer)
2 job/hr + 3 job/hr 5 job/hr
