Условие:

29 + 17
61 − 28
90 − 16
56 + 34

29 + 17 = (20 + 10) + (9 + 7) = 30 + 16 = 46
 
61 − 28 = (50 + 11) − (20 + 8) = (50 − 20) + (11 − 8) = 30 + 3 = 33
 
90 − 16 = 90 − (10 + 6) = (90 − 10) − 6 = 80 − 6 = 74
 
56 + 34 = (50 + 30) + (6 + 4) = 80 + 10 = 90