№4. Выполните действия: a) $6\frac{2}{25} + 7\frac{23}{25} - 4\frac{1}{40}$; б) $20\frac{1}{19} - 4\frac{12}{19} - 5\frac{17}{19}$; в) $(6\frac{5}{24} + 7\frac{19}{24}) - 6\frac{19}{23} - 5\frac{9}{23}$.

Решение: a) $6\frac{2}{25} + 7\frac{23}{25} - 4\frac{1}{40} = (6 + 7 - 4) + (\frac{2}{25} + \frac{23}{25} - \frac{1}{40}) = 9 + (\frac{25}{25} - \frac{1}{40}) = 9 + 1 - \frac{1}{40} = 10 - \frac{1}{40} = 9\frac{40}{40} - \frac{1}{40} = 9\frac{39}{40}$. б) $20\frac{1}{19} - 4\frac{12}{19} - 5\frac{17}{19} = (20 - 4 - 5) + (\frac{1}{19} - \frac{12}{19} - \frac{17}{19}) = 11 + \frac{1 - 12 - 17}{19} = 11 + \frac{-28}{19} = 11 - \frac{28}{19} = 11 - 1\frac{9}{19} = 10\frac{19}{19} - 1\frac{9}{19} = 9\frac{10}{19}$. в) $(6\frac{5}{24} + 7\frac{19}{24}) - 6\frac{19}{23} - 5\frac{9}{23} = (6 + 7) + (\frac{5}{24} + \frac{19}{24}) - (6 + 5) - (\frac{19}{23} + \frac{9}{23}) = 13 + \frac{24}{24} - 11 - \frac{28}{23} = 13 + 1 - 11 - \frac{28}{23} = 3 - \frac{28}{23} = 3 - 1\frac{5}{23} = 2\frac{23}{23} - 1\frac{5}{23} = 1\frac{18}{23}$. Ответы: a) $9\frac{39}{40}$; б) $9\frac{10}{19}$; в) $1\frac{18}{23}$.