the sum would be
1/4 + 3/5 = 5/20 + 12/20 = 17/20
plus indicates that it is a sum (not a difference): 1/4 + 2/4 = 3/4. You can just add the numerators, since the denominators are the same.
The sum of 1/8 and 3/8 is 1/2
3/8 + 1/8 = 1/2
The sum of 1/45 and 4/45 is 1/9
The sum of 1/6 and 1/4 is 5/12.
The sum of 7/9 and 1/2 is 1 5/18
The sum of 5/6 and 1/4 is 1 1/12.
The sum of 1/8 and 3/4 is 7/8.
The sum of 3/4 and 1/9 is 31/36
The sum of 3/8 and 1/4 is 5/8.
The sum of 1/6 and 4/9 is 11/18.
The sum of 5/8 and 1/13 is 73/104