Вычислите устно.
38 + 7
380 + 70
24 + 16
240 + 160
50 − 3
500 − 30
42 − 24
420 − 240
38 + 7 = 45
380 + 70 = 450
24 + 16 = 40
240 + 160 = 400
50 − 3 = 47
500 − 30 = 470
42 − 24 = 18
420 − 240 = 180
Вычисления:
38 + 7 = 38 + (2 + 5) = (38 + 2) + 5 = 40 + 5 = 45
380 + 70 = 380 + (20 + 50) = (380 + 20) + 50 = 400 + 50 = 450
24 + 16 = (20 + 4) + (10 + 6) = (20 + 10) + (4 + 6) = 30 + 10 = 40
240 + 160 = (200 + 40) + (100 + 60) = (200 + 100) + (60 + 40) = 300 + 100 = 400
50 − 3 = (40 + 10) − 3 = 40 + (10 − 3) = 40 + 7 = 47
500 − 30 = (400 + 100) − 30 = 400 + (100 − 30) = 400 + 70 = 470
42 − 24 = 42 − (24 + 2) = (42 − 22) − 2 = 20 − 2 = 18
420 − 240 = 420 − (220 + 20) = (420 − 220) − 20 = 200 − 20 = 180
