**Решение:**
1) -1/4 + 3/5 = -5/20 + 12/20 = 7/20
2) 9/11 + (-2/5) = 45/55 - 22/55 = 23/55
3) 20/21 + 3/7 = 20/21 + 9/21 = 29/21 = 1 8/21
4) -8 9/14 + 3 3/14 = -5 6/14 = -5 3/7
5) 7 5/12 + (-3 7/24) = 7 10/24 - 3 7/24 = 4 3/24 = 4 1/8
6) -6 11/12 + (-8 13/18) = -6 33/36 - 8 26/36 = -14 59/36 = -15 23/36
7) -5 12/35 + 10 = 4 23/35
8) -11 9/15 + 8 2/15 = -3 7/15
9) -3 1/12 + 1/6 = -3 1/12 + 2/12 = -2 11/12
10) 3 5/6 + (-6 3/4) = 3 10/12 - 6 9/12 = -2 11/12
11) 9 1/4 + (-5 3/4) = 3 2/4 = 3 1/2
12) -3 5/6 + (-2 1/12) = -3 10/12 - 2 1/12 = -5 11/12
**Ответы:**
1) 7/20
2) 23/55
3) 1 8/21
4) -5 3/7
5) 4 1/8
6) -15 23/36
7) 4 23/35
8) -3 7/15
9) -2 11/12
10) -2 11/12
11) 3 1/2
12) -5 11/12