А) (1/2 + 0,125-1/6)(6,4:80/3)+1/8 б) (6 2/3+2 4/15+5 1/2):1/15-30:5/28 в) (3

А) Запишем десятичные дроби в виде обыкновенных:
0,125 = 125/1000 = 1/8;
6,4 = 6 4/10 = 6 2/5;
1) 1/2 + 1/8 = 4/8 + 1/8 = 5/8;
2) 5/8 - 1/6 = 3*5/24 - 4*1/24 = 15/24 - 4/24 = 11/24;
3) 6 2/5 : 80/3 = 32/5 * 3/80 = 2/5 * 3/5 = 6/25;
4) 11/24 * 6/25 = 11/4 * 1/25 = 11/100;
5) 11/100 + 1/8 = 22/200 + 25/200 = 47/200.
Ответ: 47/200.
Б) 1) 6 2/3 + 2 4/15 = 6 5*2/15 + 2 4/15 = 6 10/15 + 2 4/15 = 8 14/15;
2) 8 14/15 + 5 1/2 = 8 2*14/30 + 5 15*1/30 = 8 28/30 + 5 15/30 = 13 43/30 = 14 13/30;
3) 14 13/30 : 1/15 = 433/30 * 15/1 = 433/2 * 1/1 = 433/2 = 216 1/2;
4) 30 : 5/28 = 30/1 * 28/5 = 6/1 * 28/1 = 168/1 = 168;
5) 216 1/2 - 168 = 48 1/2.
Ответ: 48 1/2.
В) 0,85 = 85/100 = 17/20;
1) 3 13/15 - 12 3/20 = 3 4*13/60 - 12 3*3/60 = 3 52/60 - 12 9/60 = 3 52/60 - 11 69/60 = - 8 17/60;
2) - 8 17/60 - 5 4/45 = - 8 3*17/180 - 5 4*4/180 = - 8 51/180 - 5 16/180 = - 13 67/180;
3) - 13 67/180 - 17/20 = - 13 67/180 - 9*17/180 = - 13 67/180 - 153/180 = - 13 220/180 = - 14 40/180 = - 14 4/18 = - 14 2/9;
4) - 14 2/9 * 3 = - 14 2/9 * 3/1 = - 14 2/3 * 1/1 = - 14 2/3.
Ответ: - 14 2/3.