Условие:

7 + 8;
17 + 8;
87 + 8;
6 + 9;
36 + 9;
56 + 9;
12 − 8;
62 − 8;
92 − 8;
14 − 6;
74 − 6;
94 − 6.

7 + 8 = 15;
17 + 8 = 10 + (7 + 8) = 10 + 15 = 25;
87 + 8 = 80 + (7 + 8) = 80 + 15 = 95;
6 + 9 = 15;
36 + 9 = 30 + (6 + 9) = 30 + 15 = 45;
56 + 9 = 50 + (6 + 9) = 50 + 15 = 65;
12 − 8 = 12 − 8 = 4;
62 − 8 = 50 + (12 − 8) = 50 + 4 = 54;
92 − 8 = 80 + (12 − 8) = 80 + 4 = 84;
14 − 6 = 14 − 6 = 8;
74 − 6 = 60 + (14 − 6) = 60 + 8 = 68;
94 − 6 = 80 + 8 = 88.