№5. Выполните действия: a) $10\frac{4}{15} + 9\frac{11}{15} - 8\frac{1}{30}$; б) $19\frac{1}{17} - 8\frac{9}{17} - 3\frac{15}{17}$; в) $(6\frac{7}{12} + 8\frac{5}{12}) - 7\frac{16}{21} - 4\frac{6}{21}$.

Решение: a) $10\frac{4}{15} + 9\frac{11}{15} - 8\frac{1}{30} = (10 + 9 - 8) + (\frac{4}{15} + \frac{11}{15} - \frac{1}{30}) = 11 + (\frac{8}{30} + \frac{22}{30} - \frac{1}{30}) = 11 + \frac{29}{30} = 11\frac{29}{30}$. б) $19\frac{1}{17} - 8\frac{9}{17} - 3\frac{15}{17} = (19 - 8 - 3) + (\frac{1}{17} - \frac{9}{17} - \frac{15}{17}) = 8 + \frac{1 - 9 - 15}{17} = 8 + \frac{-23}{17} = 8 - \frac{23}{17} = 8 - 1\frac{6}{17} = 7\frac{17}{17} - 1\frac{6}{17} = 6\frac{11}{17}$. в) $(6\frac{7}{12} + 8\frac{5}{12}) - 7\frac{16}{21} - 4\frac{6}{21} = (6 + 8) + (\frac{7}{12} + \frac{5}{12}) - (7 + 4) - (\frac{16}{21} + \frac{6}{21}) = 14 + \frac{12}{12} - 11 - \frac{22}{21} = 14 + 1 - 11 - \frac{22}{21} = 4 - \frac{22}{21} = 4 - 1\frac{1}{21} = 3\frac{21}{21} - 1\frac{1}{21} = 2\frac{20}{21}$. Ответы: a) $11\frac{29}{30}$; б) $6\frac{11}{17}$; в) $2\frac{20}{21}$.