10 6/15 - 9 8/20 = 10 2/3 - 9 2/5 = (10 + 2/3) - (9 + 2/5) = 10 + 2/3 - 9 - 2/5 = 1 + 2/3 - 2/5 = 1 + (10/15) - (6/15) = 1 + 4/15 = 1 4/15

7 33/87 - 6 22/58 = 7 11/29 - 6 11/29 = (7 + 11/29) - (6 + 11/29) = 7 + 11/29 - 6 - 11/29 = 1 + 11/29 - 11/29 = 1