In 5 hours Matt will finish 5/12 of the job and Jeff will finish 5/J of the job where J is the number of hours J would require to do it alone. Together they finish 7/8 of the job in 5 hours, i.e. (in math terms):
(5/12) + (5/J) = 7/8
5/J= (7/8) - (5/12) = 11/24
5/J = 11/24 => J/5 = 24/11 => J = 120/11 = 10 10/11 hours for Jeff to do the whole job by himself.
You should that this answer satisfies the specifications in the original problem.