15) \(\dfrac{1}{2}-\dfrac{9}{25}\)

Общий знаменатель дробей — 50: \(\dfrac{1}{2}=\dfrac{25}{50}\), \(\dfrac{9}{25}=\dfrac{18}{50}\).

\(\dfrac{25}{50}-\dfrac{18}{50}=\dfrac{7}{50}\).

Ответ: \(\dfrac{7}{50}\).