2 5 12.5-4- 7 7

12. $$5\frac{2}{7}-4\frac{5}{7} = (5-4) + (\frac{2}{7} - \frac{5}{7}) = 1 - \frac{3}{7} = \frac{7}{7} - \frac{3}{7} = \frac{4}{7}$$.

Ответ: $$\frac{4}{7}$$