To solve these expressions, we need to find a common denominator for each set of fractions.

1) 2/3 + 1/2
Common denominator: 6
(4/6) + (3/6) = 7/6

2) 1/4 - 2/15
Common denominator: 60
(15/60) - (8/60) = 7/60

3) 5/12 - 2/5
Common denominator: 60
(25/60) - (24/60) = 1/60

4) 5/6 - 1/4
Common denominator: 12
(10/12) - (3/12) = 7/12

Therefore, the final answers are:
1) 7/6
2) 7/60
3) 1/60
4) 7/12