Условие:

20 + 4 + 4 + 4
36 − 6 − 6 − 6
40 + 5 + 5 + 5
70 + 3 + 3 + 3
80 − 2 − 2 − 2
90 − 7 − 7 − 7

20 + 4 + 4 + 4 = 20 + (4 + 4 + 4) = 20 + (8 + 2 + 2) = 20 + (10 + 2) = (20 + 10) + 2 = 30 + 2 = 32
 
36 − 6 − 6 − 6 = 30 − 6 − 6 = 30 − (6 + 6) = 30 − 12 = 30 − 10 − 2 = 20 − 2 = 18
 
40 + 5 + 5 + 5 = 40 + (5 + 5 + 5) = 40 + (10 + 5) = (40 + 10) + 5 = 50 + 5 = 55
 
70 + 3 + 3 + 3 = 70 + (3 + 3 + 3) = 70 + 9 = 79
 
80 − 2 − 2 − 2 = 80 − (2 + 2 + 2) = 80 − 6 = (70 + 10) − 6 = 70 + (10 − 6) = 70 + 4 = 74
 
90 − 7 − 7 − 7 = 90 − (7 + 7 + 7) = 90 − (14 + 7) = 90 − (14 + 6 + 1) = 90 − (20 + 1) = (90 − 20) − 1 = 70 − 1 = 69