Приведем подобные слагаемые:
1) \( 8x + 21x = (8 + 21)x = 29x \)
2) \( 78a - 49a = (78 - 49)a = 29a \)
3) \( 326y + 99y = (326 + 99)y = 425y \)
4) \( 15,3x - 8,8x = (15,3 - 8,8)x = 6,5x \)
5) \( 9,8b + b = (9,8 + 1)b = 10,8b \)
6) \( 20,6m - m = (20,6 - 1)m = 19,6m \)
7) \( 31t - 9,1t = (31 - 9,1)t = 21,9t \)
8) \( 2y + 19y + y = (2 + 19 + 1)y = 22y \)
9) \( 6x - 5x + x = (6 - 5 + 1)x = 2x \)
10) \( 7b + b - 8b = (7 + 1 - 8)b = 0b = 0 \)
Ответ: 1) 29x; 2) 29a; 3) 425y; 4) 6,5x; 5) 10,8b; 6) 19,6m; 7) 21,9t; 8) 22y; 9) 2x; 10) 0.