А) 3-5+9+9-21+7-1=..... б) 56+4-30+20=..... в) 100+900-50-500+50=...... г) 20+70-60+40=......

Первый пример.

3 - 5 + 9 + 9 - 21 + 7 - 1 = 1;

3 - 5 = -2;

-2 + 9 = 7;

7 + 9 = 16;

16 - 21 = -5;

-5 + 7 = 2;

2 - 1 = 1;

2-ой пример.

56 + 4 - 30 + 20 = 50;

56 + 4 = 60;

60 - 30 = 30;

30 + 20 = 50;

3-ий пример.

100 + 900 - 50 - 500 + 50 = 500;

100 + 900 = 1000;

1000 - 50 = 950;

950 + 500 = 450;

450 + 50 = 500;

4-ый пример.

20 + 70 - 60 + 40 = 70;

20 + 70 = 90;

90 - 60 = 30;

30 + 40 = 70;