2/9 + 1/9 = 3/9 = 1/3

3/5 + 2/7 = (21/35) + (10/35) = 31/35

11/15 - 3/5 = (11/15) - (9/15) = 2/15

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