Условие:

Вычислите устно.
20 * 5
490 : 7
100 * 10
290 – 20
975 – 70
444 – 44
28 + 4
30 – 6
7 * 80
360 + 40
56 + 7
408 : 4

20 * 5 = 100
490 : 7 = 70
100 * 10 = 1 000
290 – 20 = 270
975 – 70 = 905
444 – 44 = 400
28 + 4 = 32
30 – 6 = 24
7 * 80 = 560
360 + 40 = 400
56 + 7 = 63
408 : 4 = 102
Вычисления:
20 * 5 = 2 * 10 * 5 = 2 * 5 * 10 = 10 * 10 = 100
490 : 7 = 49 * 10 : 7 = 49 : 7 * 10 = 7 * 10 = 70
100 * 10 = 1 000
290 – 20 = 200 + 90 − 20 = 200 + 70 = 270
975 – 70 = 900 + 75 − 70 = 900 + 5 = 905
444 – 44 = 400 + 44 − 44 = 400
28 + 4 = 28 + 2 + 2 = 30 + 2 = 32
30 – 6 = 20 + 10 − 6 = 20 + 4 = 24
7 * 80 = 7 * 8 * 10 = 56 * 10 = 560
360 + 40 = 300 + 60 + 40 = 300 + 100 = 400
56 + 7 = 56 + 4 + 3 = 60 + 3 = 63
408 : 4 = (400 + 8) : 4 = 400 : 4 + 8 : 4 = 4 * 100 : 4 + 2 = 100 + 2 = 102