4. a) $$\frac{3}{5} - \frac{4}{5}$$; б) $$\frac{6}{7} - \frac{8}{7}$$; в) $$\frac{-1}{12} - (\frac{-5}{12})$$;

a) $$\frac{3}{5} - \frac{4}{5} = \frac{3-4}{5} = \frac{-1}{5} = -\frac{1}{5}$$ Ответ: $$-\frac{1}{5}$$

б) $$\frac{6}{7} - \frac{8}{7} = \frac{6-8}{7} = \frac{-2}{7} = -\frac{2}{7}$$ Ответ: $$-\frac{2}{7}$$

в) $$\frac{-1}{12} - (\frac{-5}{12}) = \frac{-1}{12} + \frac{5}{12} = \frac{-1+5}{12} = \frac{4}{12} = \frac{1}{3}$$ Ответ: $$\frac{1}{3}$$