61 + 30 = 1 + 60 + 30 = 1 + 90 = 91
 
76 − 4 = (70 + 6) − 4 = 70 + (6 − 4) = 70 + 2 = 72
 
40 + 17 = 40 + 10 + 7 = 50 + 7 = 57
 
50 − 23 = 50 − (20 + 3) = 50 − 20 − 3 = 30 − 3 = 27
 
84 + 6 = 80 + 4 + 6 = 80 + 10 = 90
 
30 − 9 = (20 + 10) − 9 = 20 + (10 − 9) = 20 + 1 = 21