a) \(\frac{2}{3} \cdot \frac{6}{7} : \frac{4}{7}\)

Решение:

1. \( \frac{2}{3} \cdot \frac{6}{7} = \frac{2 \cdot 6}{3 \cdot 7} = \frac{12}{21} \)
2. \( \frac{12}{21} : \frac{4}{7} = \frac{12}{21} \cdot \frac{7}{4} = \frac{12 \cdot 7}{21 \cdot 4} \)
3. Сокращаем: \( \frac{\cancel{12}^3 \cdot \cancel{7}^1}{\cancel{21}^3 \cdot \cancel{4}^1} = \frac{3 \cdot 1}{3 \cdot 1} = 1 \)

Ответ: 1