Since the two times are different you have to find their average first.
The average of 8 and 10 is 9. You can figure this out by adding the two numbers together and then dividing by the number of numbers you're finding the average to.
Now you divide the average by the variable (the number of people working on the job would be the variable I think, if not, its not important anyway but you still divide by the number of people working.)
So 9÷2= 4.5
Therefore, it would take 4.5 hours for them to complete the job if they worked together.