Чтоб решить эти примеры, нужно свести дроби к общему знаменателю. Так уж и быть, помогу :)

1/3 - 1/6 = 2/6 - 1/6 = 1/6

3/20 + 3/4 = 3/20 + 15/20 = 18/20 = 9/10

7/9 - 2/3 = 7/9 - 6/9 = 1/9

1/4 + 1/5 = 5/20 + 4/20 = 9/20

2/5 + 4/15 = 6/15 + 4/15 = 10/15 = 2/3

1/3 + 1/4 = 4/12 + 3/12 = 7/12

2/3 - 2/5 = 10/15 - 6/15 = 4/15

1/5 - 1/6 = 6/30 - 5/30 = 1/30

1/2 + 1/4 = 2/4 + 1/4 = 3/4

7/6 - 1 = 1 1/6 - 1 = 1/6

1/2 + 2/7 = 7/14 + 4/14 = 11/14

1/2 - 1/3 = 3/6 - 2/6 = 1/6

3/4 + 1/8 = 6/8 - 1/8 = 5/8

5/6 - 7/12 = 10/12 - 7/12 = 3/12 = 1/4

2/5 + 3/10 = 4/10 + 3/10 = 7/10