87/13-7 12/13 6 15/28-(2 15/18-1 17/28) 6 34/39+7 38/39+5 37/39 68 32/67+10 10/67+31 25/67

8 7/13 - 7 12/13 = 7 20/13 - 7 12/13 = 8/13
6 15/28 - (2 15/28 - 1 17/28) = 6 15/28 - 2 15/28 + 1 17/28 = 5 17/28
6 34/39 + 7 38/39 + 5 37/39 = 18 109/39 = 20 31/39
68 32/67 + 10 10/67 + 31 25/67 = 109 67/67 = 110