6. Выполните действия: 1) $$6\frac{8}{9}-3\frac{1}{2}+3\frac{1}{12}$$; 3) $$(19\frac{5}{21}-8\frac{3}{14})-6\frac{5}{7}$$; 2) $$8\frac{7}{9}+6\frac{2}{15}-4,3$$; 4) $$(19-8\frac{7}{8})-(4\frac{5}{6}-2\frac{8}{9})$$.

Решим примеры по действиям: 1) $$6\frac{8}{9}-3\frac{1}{2}+3\frac{1}{12}$$; $$\frac{62}{9} - \frac{7}{2} + \frac{37}{12} = \frac{62 \cdot 4}{9 \cdot 4} - \frac{7 \cdot 18}{2 \cdot 18} + \frac{37 \cdot 3}{12 \cdot 3} = \frac{248}{36} - \frac{126}{36} + \frac{111}{36} = \frac{248 - 126 + 111}{36} = \frac{233}{36} = 6\frac{17}{36}$$. 2) $$8\frac{7}{9}+6\frac{2}{15}-4,3$$; $$\frac{79}{9} + \frac{92}{15} - 4,3 = \frac{79 \cdot 5}{9 \cdot 5} + \frac{92 \cdot 3}{15 \cdot 3} - \frac{43}{10} = \frac{395}{45} + \frac{276}{45} - \frac{43}{10} = \frac{395 \cdot 2}{45 \cdot 2} + \frac{276 \cdot 2}{45 \cdot 2} - \frac{43 \cdot 9}{10 \cdot 9} = \frac{790}{90} + \frac{552}{90} - \frac{387}{90} = \frac{790 + 552 - 387}{90} = \frac{955}{90} = \frac{191}{18} = 10\frac{11}{18}$$. 3) $$(19\frac{5}{21}-8\frac{3}{14})-6\frac{5}{7}$$; $$(19\frac{5}{21}-8\frac{3}{14})-6\frac{5}{7} = (\frac{404}{21} - \frac{115}{14}) - \frac{47}{7} = (\frac{404 \cdot 2}{21 \cdot 2} - \frac{115 \cdot 3}{14 \cdot 3}) - \frac{47}{7} = (\frac{808}{42} - \frac{345}{42}) - \frac{47}{7} = \frac{463}{42} - \frac{47}{7} = \frac{463}{42} - \frac{47 \cdot 6}{7 \cdot 6} = \frac{463}{42} - \frac{282}{42} = \frac{463 - 282}{42} = \frac{181}{42} = 4\frac{13}{42}$$. 4) $$(19-8\frac{7}{8})-(4\frac{5}{6}-2\frac{8}{9})$$; $$(19-8\frac{7}{8})-(4\frac{5}{6}-2\frac{8}{9}) = (19 - \frac{71}{8}) - (\frac{29}{6} - \frac{26}{9}) = (\frac{19 \cdot 8}{8} - \frac{71}{8}) - (\frac{29 \cdot 3}{6 \cdot 3} - \frac{26 \cdot 2}{9 \cdot 2}) = (\frac{152}{8} - \frac{71}{8}) - (\frac{87}{18} - \frac{52}{18}) = \frac{81}{8} - \frac{35}{18} = \frac{81 \cdot 9}{8 \cdot 9} - \frac{35 \cdot 4}{18 \cdot 4} = \frac{729}{72} - \frac{140}{72} = \frac{729 - 140}{72} = \frac{589}{72} = 8\frac{13}{72}$$. Ответ: 1) $$6\frac{17}{36}$$; 2) $$10\frac{11}{18}$$; 3) $$4\frac{13}{42}$$; 4) $$8\frac{13}{72}$$.