This belongs in the mathmatics catagory, but:
5 men do 3 units of work in 3 hrs.
5 men do 1 unit of work in 1 hr.
1 man does 1/5 unit of work in 1 hr.
2 men do 2/5 units of work in 1 hr.
Therefore 2 men will take 5/2 hours to dig 1 hole and take 5/4 hrs to dig half a hole.
So the answer you're looking for is 1 hr 15 mins
