775. Выполните действия: 1) $$8 + \frac{4}{21}$$; 2) $$5\frac{16}{19} + 3\frac{5}{19}$$; 3) $$7\frac{7}{16} - 3\frac{3}{16}$$; 4) $$10\frac{12}{17} + 5\frac{4}{17} - 3\frac{3}{17}$$.

1) $$8 + \frac{4}{21} = 8\frac{4}{21}$$

2) $$5\frac{16}{19} + 3\frac{5}{19} = (5 + 3) + (\frac{16}{19} + \frac{5}{19}) = 8 + \frac{16 + 5}{19} = 8 + \frac{21}{19} = 8 + 1\frac{2}{19} = 9\frac{2}{19}$$

3) $$7\frac{7}{16} - 3\frac{3}{16} = (7 - 3) + (\frac{7}{16} - \frac{3}{16}) = 4 + \frac{7 - 3}{16} = 4 + \frac{4}{16} = 4 + \frac{1}{4} = 4\frac{1}{4}$$

4) $$10\frac{12}{17} + 5\frac{4}{17} - 3\frac{3}{17} = (10 + 5 - 3) + (\frac{12}{17} + \frac{4}{17} - \frac{3}{17}) = 12 + \frac{12 + 4 - 3}{17} = 12 + \frac{13}{17} = 12\frac{13}{17}$$

Ответ: 1) $$8\frac{4}{21}$$; 2) $$9\frac{2}{19}$$; 3) $$4\frac{1}{4}$$; 4) $$12\frac{13}{17}$$