What is the reasonable estimate for the problem 3 3/4 - 2/5

A reasonable estimate could be turning 2/5 into 2/4 (1/2) then subtracting it to give you 3 3/4 - 2/4 = 3 1/4.

To prove this estimate is reasonable, the actual answer to the problem is 3 7/20, which is pretty close to 3 1/4! (Could be written as 3 5/20)