Mutt and Jeff need to paint a fence. Mutt can do the job alone 8 hours faster than Jeff. If together they work for 9 hours and finish only 4/5 of the job, how long would Jeff need to do the job alone?

Your answer must be a number. No arithmetic operations are allowed.

It would take Jeff _____ hours to do the job alone.

(i have already submitted this questions and was given an answer of 5 hours, but this answer is incorrect. Could you please refigure)