John can build a fence in two days; bill can build the same fence in five days. if john and bill worked together, how long would it take them to build the fence?

-- John takes 2 days to build a fence.  He builds 1/2 of a fence each day.

-- Bill takes 5 days to build a fence.  He builds 1/5 of a fence each day.

-- Working together, they build (1/2 + 1/5) of a fence each day.

Before we can add (1/2 + 1/5), we need a common denominator.
The least common multiple of 2 and 5 is 10.
1/2 = 5/10
1/5 = 2/10
(1/2 + 1/5) = (5/10 + 2/10) = 7/10 fence  <==

Working together, they build at the rate of 7/10 fence per day.

==>  Or  10/7 day per fence.

10/7 day  =  1-3/7 day 

If their work day is 8 hours, then 1-3/7 day is ...

            1 day  3 hours  25 minutes  42.9 seconds of work per fence.