Условие:

Вычисли.
9 + 8;
16 − 7;
36 + 23;
15 + 45;
17 + 50;
69 − 40;
87 − 43;
70 − 9.

9 + 8 = 9 + (1 + 7) = (9 + 1) + 7 = 10 + 7 = 17;
16 − 7 = 16 − (6 + 1) = 16 − 6 − 1 = 10 − 1 = 9;
36 + 23 = (30 + 20) + (6 + 3) = 50 + 9 = 59;
15 + 45 = (10 + 40) + (5 + 5) = 50 + 10 = 60;
17 + 50 = (10 + 7) + 50 = (10 + 50) + 7 = 60 + 7 = 67;
69 − 40 = (9 + 60) − 40 = 9 + (60 − 40) = 9 + 20 = 29;
87 − 43 = (80 − 40) + (7 − 3) = 40 + 4 = 44;
70 − 9 = (60 + 10) − 9 = 60 + (10 − 9) = 60 + 1 = 61.