5. Решите уравнение: в) 1 2/7 : 5 1/7 = 2/3 : x.

$$1\frac{2}{7} : 5\frac{1}{7} = \frac{2}{3} : x$$

$$\frac{9}{7} : \frac{36}{7} = \frac{2}{3} : x$$

$$\frac{\frac{9}{7}}{\frac{36}{7}} = \frac{\frac{2}{3}}{x}$$, $$\frac{9}{36} = \frac{2}{3x}$$

$$\frac{1}{4} = \frac{2}{3x}$$, $$3x = 8$$, $$x = \frac{8}{3}$$

$$x = 2\frac{2}{3}$$

Ответ: $$2\frac{2}{3}$$.