Raul can complete a job in 8 hours. Tom can complete the same job in 6 hrs.

How long will it take the two of them to complete the job working together?

in 1 hour Raul completes 1/8th of the job
in 1 hour Tom completes 1/6th of the job
in 1 hour Tom and Raul together complete (1/8 + 1/6)th of the job = (3+4)/24 = 7/24
therefore number of hours it takes both to finish = 24/7 = 3.429 hours

(8+6)/2 = 7 is the average time they would use individually.
Working together they would use 7/2 = 3.5 hours.

Seven hours