1/3 - 1/6 =... 3/21 - 1/7 =... 1/6 - 1/9 =...

1. 1/3 - 1/6 = 2/6 - 1/6 = 1/6
2. 3/21 - 1/7 = 3/21 - 3/21 = 0
3. 1/6 - 1/9 = 3/18 - 2/18 = 1/18

(sorry ya kalah salah)