2 1/7a + 3 9/14a = 11 1/7 - 1 1/21 - 2 3/14
2 2/14a + 3 9/14 = 11 6/42 - 1 2/42 - 2 9/42
5 11/14a = 9 39/42 или 9 13/14
a = 9 39/42 : 5 11/14
a = 1 58/81.
1)2801×640=1792640
2)1792640-1789631=3009
3)95823:189=507
4)775-507=268
5)1630314:798=2043
6)3009×268=806412
7)2043+268=2311
Ответ:2311
12 целых 8/15-12целых=8/15