60 + 7 = 67
67 - 7 = 60
67 - 60 = 7
50 + 4 = 54
54 - 4 = 50
54 - 50 = 4
30 + 6 = 36
36 - 6 = 30
36 - 30 = 6
a)
28 = 20 + 8
39 = 30 + 9
54 = 50 + 4
б)
1) 20 + (3 + 5) = 20 + 8 = 28
2) 30 + (7 - 2) = 30 + 5 = 35
3) 40 + (8 - 2) = 40 + 6 = 46
4) 30 + (2 + 7) = 30 + 9 = 39
5) 50 + (6 - 4) = 50 + 2 = 52
6) 20 + (2 + 6) = 20 + 8 = 28
7) 50 + (3 + 6) = 50 + 9 = 59
8) 40 + (9 - 5) = 40 + 4 = 44
9) 70 + (7 - 6) = 70 + 1 = 71
10) 80 + (3 - 2) = 80 + 1 = 81
11) 70 + (5 - 4) = 70 + 1 = 71
12) 30 + (5 + 4) = 30 + 9 = 39