a) \( 1\frac{5}{7} : x = \frac{6}{7} : 2 \)

Решение:

1. \( \frac{12}{7} : x = \frac{6}{7} : 2 \)
2. \( \frac{12}{7} : x = \frac{6}{7} \cdot \frac{1}{2} \)
3. \( \frac{12}{7} : x = \frac{3}{7} \)
4. \( x = \frac{12}{7} : \frac{3}{7} \)
5. \( x = \frac{12}{7} \cdot \frac{7}{3} \)
6. \( x = 4 \)

Ответ: \( x = 4 \).