13) $$rac{1}{13b} \cdot \frac{26}{3} =$$

$$\frac{1}{13b} \cdot \frac{26}{3} = \frac{1 \cdot 26}{13b \cdot 3} = \frac{26}{39b} = \frac{2 \cdot 13}{3 \cdot 13 \cdot b} = \frac{2}{3b}$$

Ответ: $$\frac{2}{3b}$$