17/21 - 1/7 = 17/21 - 3/21 = 14/21 = 2/3
5/12 + 14/15 = 25/60 + 56/60 = 81/60 = 27/20
7/12 - 2/15 = 35/60 - 8/60 = 27/60 = 9/20
8/15 - (5/21+1/7) = 8/15 - (5/21 + 3/21) = 8/15 - 8/21 = 56/105 - 40/105 = 16/105
(3/16 + 5/12) - 3/8 = (9/48 + 20/48) - 3/8 = 29/48 - 3/8 = 1/2 - 3/8 = 4/8 - 3/8 = 1/2