Условие:

80 − 8
100 − 7
40 − 4
70 − 2

80 − 8 = (70 + 10) − 8 = 70 + (10 − 8) = 70 + 2 = 72
100 − 7 = (90 + 10) − 7 = 90 + (10 − 7) = 90 + 3 = 93
40 − 4 = (30 + 10) − 4 = 30 + (10 − 4) = 30 + 6 = 36
70 − 2 = (60 + 10) − 2 = 60 + (10 − 2) = 60 + 8 = 68