781°. Вычислите: 1) 6 4/9 + 3 5/9; 2) 10 11/19 + 5 14/19; 3) 1 5/8 + 3 7/8; 4) 1 − 3/11; 5) 1 − 13/40; 6) 4 − 1 4/7; 7) 10 − 9 3/10; 8) 5 2/7 − 2 5/7; 9) 14 6/20 − 8 12/20; 10) 8 3/14 − 5 9/14; 11) 7 10/21 − 4 16/21; 12) 14 8/31 − 6 8/31.

1. \(6\frac{4}{9}+3\frac{5}{9}=9\frac{9}{9}=10\).
2. \(10\frac{11}{19}+5\frac{14}{19}=15\frac{25}{19}=17\frac{6}{19}\).
3. \(1\frac{5}{8}+3\frac{7}{8}=4\frac{12}{8}=5\frac{1}{2}\).
4. \(1-\frac{3}{11}=\frac{8}{11}\).
5. \(1-\frac{13}{40}=\frac{27}{40}\).
6. \(4-1\frac{4}{7}=3\frac{7}{7}-1\frac{4}{7}=2\frac{3}{7}\).
7. \(10-9\frac{3}{10}=\frac{7}{10}\).
8. \(5\frac{2}{7}-2\frac{5}{7}=4\frac{9}{7}-2\frac{5}{7}=2\frac{4}{7}\).
9. \(14\frac{6}{20}-8\frac{12}{20}=5\frac{14}{20}=5\frac{7}{10}\).
10. \(8\frac{3}{14}-5\frac{9}{14}=2\frac{8}{14}=2\frac{4}{7}\).
11. \(7\frac{10}{21}-4\frac{16}{21}=2\frac{15}{21}=2\frac{5}{7}\).
12. \(14\frac{8}{31}-6\frac{8}{31}=8\).

Ответ: 1) 10; 2) \(17\frac{6}{19}\); 3) \(5\frac{1}{2}\); 4) \(\frac{8}{11}\); 5) \(\frac{27}{40}\); 6) \(2\frac{3}{7}\); 7) \(\frac{7}{10}\); 8) \(2\frac{4}{7}\); 9) \(5\frac{7}{10}\); 10) \(2\frac{4}{7}\); 11) \(2\frac{5}{7}\); 12) 8.