20. $$5\frac{1}{6} : x = -31$$

$$\begin{aligned} 5\frac{1}{6} : x &= -31 \\ \frac{31}{6} : x &= -31 \\ x &= \frac{31}{6} : (-31) \\ x &= \frac{31}{6} \cdot \frac{1}{-31} \\ x &= -\frac{1}{6} \end{aligned}$$ Ответ: x = -1/6