21. Решите уравнение: а) (у − 8 12/19) + 1 7/19 = 6 2/19; б) (12 5/13 + у) − 9 9/13 = 7 7/13; в) (х − 3 13/21) + 2 10/21 = 7 2/21.

1. \(y-8\frac{12}{19}+1\frac{7}{19}=6\frac{2}{19}\).
   \(y=6\frac{2}{19}+8\frac{12}{19}-1\frac{7}{19}=13\frac{7}{19}\).
2. \(12\frac{5}{13}+y-9\frac{9}{13}=7\frac{7}{13}\).
   \(y=7\frac{7}{13}+9\frac{9}{13}-12\frac{5}{13}=4\frac{11}{13}\).
3. \(x-3\frac{13}{21}+2\frac{10}{21}=7\frac{2}{21}\).
   \(x=7\frac{2}{21}+3\frac{13}{21}-2\frac{10}{21}=8\frac{5}{21}\).

Ответ: а) \(13\frac{7}{19}\); б) \(4\frac{11}{13}\); в) \(8\frac{5}{21}\).