12. 5 -4

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: $$\frac{4}{7}$$