1)  (7/15-1/6)+2/5 = (14/30-5/30)+12/30 =21/30

2)  (3/8-1/9)+25/36 = (27/72-8/72)+50/72 = 69/72 =23/24

3)  (8/9-5/6)+2/3 = (16/18-15/18)+12/18 = 13/18

4)   (7/8-13/20)+9/10 = (35/40-26/40)+36/40 = 45/40 = 9/8 = 1 1/8

5)   (7/18-1/12)+5/6 = (14/36-3/36)+30/36 = 41/36 = 1 5/36

6)   (3/4-8/15)+17/20 = (45/60-32/60)+51/60 = 64/60 = 16/15 = 1 1/15