Question 940079
John can build 1/5 of the wall per hour.
Mark can build 1/8 of the wall per hour. 
Working together, each hour they can build
1/5+1/8=8/40+5/40=13/40
of the wall.
So, working together, it should take them
{{{2/((13/40))=1*(40/13)=40/13=3&1/13}}} hours.
Since 1/13 of an hour is 60/13 minutes, 
which is a little more than 4.6 minutes, but less than 5 minutes,
I think they should estimate that it will take them
3 hours and 5 minutes.