7 1 -- -- = 10 2

$$\frac{7}{10} - \frac{1}{2} = \frac{7}{10} - \frac{1 \cdot 5}{2 \cdot 5} = \frac{7}{10} - \frac{5}{10} = \frac{2}{10} = \frac{1}{5}$$

Ответ: $$\frac{1}{5}$$