\(\frac{12}{15} = \frac{4 \cdot 3}{5 \cdot 3} = \frac{4}{5}\)
\(\frac{x}{y} \cdot \frac{a}{a} = \frac{x}{y} \cdot 1 = \frac{x}{y}\)
\(\frac{4}{5} \cdot \frac{x}{y} = \frac{4x}{5y}\)
Ответ: \(\frac{4x}{5y}\)