Вычислите: а) \(\frac{5}{7}\cdot\frac{21}{25}\cdot\frac{5}{9}\).

Сократим дроби: \(\frac{21}{7}=3\), \(\frac{5}{25}=\frac{1}{5}\).

\[\frac{5}{7}\cdot\frac{21}{25}\cdot\frac{5}{9}=\frac{1\cdot3\cdot5}{5\cdot9}=\frac{15}{45}=\frac{1}{3}.\]

Ответ: \(\frac{1}{3}\).