40 − 8 = (30 + 10) − 8 = 30 + (10 − 8) = 30 + 2 = 32
30 − 7 = (20 + 10) − 7 = 20 + (10 − 7) = 20 + 3 = 23
50 − 15 = 50 − (10 + 5) = (50 − 10) − 5 = 40 − 5 = 35
60 − 12 = 60 − (10 + 2) = (60 − 10) − 2 = 50 − 2 = 48
70 − 12 = 70 − (10 + 2) = (70 − 10) − 2 = 60 − 2 = 58
80 − 12 = 80 − (10 + 2) = (80 − 10) − 2 = 70 − 2 = 68