Условие:

Вычисли удобным способом.
8 + 39 + 12 + 21;
6 + 17 + 14 + 3;
28 + 27 + 2 + 3;
65 + 7 + 5 + 23;
38 + 19 + 2 + 1;
56 + 25 + 4 + 5.

8 + 39 + 12 + 21 = (8 + 12) + (39 + 21) = 20 + 60 = 80;
6 + 17 + 14 + 3 = (6 + 14) + (17 + 3) = 20 + 20 = 40;
28 + 27 + 2 + 3 = (28 + 2) + (27 + 3) = 30 + 30 = 60;
65 + 7 + 5 + 23 = (65 + 5) + (7 + 23) = 70 + 30 = 100;
38 + 19 + 2 + 1 = (38 + 2) + (19 + 1) = 40 + 20 = 60;
56 + 25 + 4 + 5 = (56 + 4) + (25 + 5) = 60 + 30 = 90.