№276.
5 7/8 + 6 3/10 = 5 35/40 + 6 12/40 = (5+6) + ( (35+12)/40) =
= 11 + 1 7/40 = 12 7/40
6 3/8 + 2 5/9 = 6 27/72 + 2 40/72 = 8 67/72
1 8/21 + 4 3/14 + 2 2/7 = 1 16/42 + 4 9/42 + 2 12/42 = 7 37/42
№279.
1 - 13/55 = (55-13)/55 = 42/55
6 - 9/20 = 5 + (20-9)/20 = 5 11/20
10 - 5 12/35 = 5 - 12/35 = 4 23/35
8 - 4 5/22 = 4 - 5/22 = 3 17/22