Условие:

Вычислите устно.
25 + 6
130 − 20
12 * 4
51 : 17
40 * 6
120 : 30
681 − 600
64 + 36
80 : 2
15 * 4
230 + 70
100 − 75

25 + 6 = 31
130 − 20 = 110
12 * 4 = 48
51 : 17 = 3
40 * 6 = 240
120 : 30 = 4
681 − 600 = 81
64 + 36 = 100
80 : 2 = 40
15 * 4 = 60
230 + 70 = 300
100 − 75 = 25
Вычисления:
25 + 6 = 25 + (5 +1) = (25 + 5) + 1 = 30 + 1 = 31
130 − 20 = 100 + (30 − 20) = 100 + 10 = 110
12 * 4 = (10 + 2) * 4 = 10 * 4 + 2 * 4 = 40 + 8 = 48
51 : 17 = 3 (методом подбора 17 * 2 = 34, 17 * 3 = 51)
40 * 6 = 4 * 6 * 10 = 24 * 10 = 240
120 : 30 = (12 * 10) : (3 * 10) = 12: 3 = 4
681 − 600 = (600 + 81) − 600 = 81
64 + 36 = 64 + (30 + 6) = (64 + 6) + 30 = 70 + 30 = 100
80 : 2 = (8 * 10) : 2 = (8 : 2) * 10 = 4 * 10 = 40
15 * 4 = (10 + 5) * 4 = 10 * 4 + 5 * 4 = 40 + 20 = 60
230 + 70 = (200 + 30) + 70 = 200 + (30 + 70) = 200 + 100 = 300
100 − 75 = 100 − (70 + 5) = (100 − 70) − 5 = 30 − 5 = 25