8 целых 1/2 +1 целая 2/5 = 8 5/10 + 1 4/10 = 9 9/10
10 целых 1/4 - 6 целых 1/5 = 10 5/20 - 6 4/20 = 4 1/20
11 целых 5/8 +8 целых 101/125 = 11 625/1000 + 8 808/1000 = 19 1433/1000=20 433/1000
21 целая 15/16 - 19 целых 3/125 = 21 1875/2000 - 19 48/2000 = 2 1827/2000