Решение:
\[ \frac{1}{2} = \frac{1 \times 25}{2 \times 25} = \frac{25}{50} \]
\[ \frac{9}{25} = \frac{9 \times 2}{25 \times 2} = \frac{18}{50} \]
\[ \frac{25}{50} - \frac{18}{50} = \frac{25 - 18}{50} = \frac{7}{50} \]
Ответ: 7/50