Условие:

Вычислите устно.
420 + 80
290 – 50
20 * 5
14 * 2
480 : 20
560 : 7
20 * 30
70 * 5
1 000 : 10
10 * 100
452 * 1
0 : 958

420 + 80 = 500
290 – 50 = 240
20 * 5 = 100
14 * 2 = 28
480 : 20 = 24
560 : 7 = 80
20 * 30 = 600
70 * 5 = 350
1 000 : 10 = 100
10 * 100 = 1 000
452 * 1 = 452
0 : 958 = 0
Вычисления:
420 + 80 = 400 + 20 + 80 = 400 +100 = 500
290 – 50 = 200 + 90 − 50 = 200 + 40 = 240
20 * 5 = 2 * 5 * 10 = 10 * 10 = 100
14 * 2 = (10 + 4) * 2 = 10 * 2 + 4 * 2 = 20 + 8 = 28
480 : 20 = 480 : 2 : 10 = 240 : 10 = 24
560 : 7 = 56 * 10 : 7 = 56 : 7 * 10 = 8 * 10 = 80
20 * 30 = 2 * 3 * 10 * 10 = 6 * 10 * 10 = 60 * 10 = 600
70 * 5 = 7 * 5 * 10 = 35 * 10 = 350
1 000 : 10 = 100 * 10 : 10 = 100
10 * 100 = 1 000
452 * 1 = 452
0 : 958 = 0