1) 3/12 + 25/30
3/12 = 1/4
25/30 = 5/6
1/4 + 5/6 = (13)/(43) + (52)/(62) = 3/12 + 10/12 = 13/12

2) 10/24 - 21/54
10/24 = 5/12
21/54 = 7/18
5/12 - 7/18 = (53)/(123) - (72)/(182) = 15/36 - 14/36 = 1/36

3) 30/54 + 22/30
30/54 = 5/9
22/30 = 11/15
5/9 + 11/15 = (55)/(95) + (113)/(153) = 25/45 + 33/45 = 58/45

4) 28/40 - 10/75
28/40 = 7/10
10/75 = 2/15
7/10 - 2/15 = (73)/(103) - (22)/(152) = 21/30 - 4/30 = 17/30

5) 12/27 + 14/21
12/27 = 4/9
14/21 = 2/3
4/9 + 2/3 = (43)/(93) + (29)/(39) = 12/27 + 18/27 = 30/27 = 10/9

6) 14/24 - 15/40
14/24 = 7/12
15/40 = 3/8
7/12 - 3/8 = (72)/(122) - (33)/(83) = 14/24 - 9/24 = 5/24

7) 12/18 - 5/60
12/18 = 2/3
5/60 = 1/12
2/3 - 1/12 = (24)/(34) - 1/12 = 8/12 - 1/12 = 7/12

8) 4/24 + 3/36
4/24 = 1/6
3/36 = 1/12
1/6 + 1/12 = (12)/(62) + 1/12 = 2/12 + 1/12 = 3/12 = 1/4