jmc

Since a terminating decimal can be written in the form of $\frac{a}{10^b}$, where $a$ and $b$ are integers, we want to rewrite our fraction with a denominator of $10^b=2^b\cdot 5^b$. $$\frac{31}{2\cdot 5^6}\cdot \frac{2^5}{2^5}=\frac{31\cdot 2^5}{10^6}=\frac{992}{10^6}.$$Because the denominator consists only of a $10^6$ term, there are a total of 6 digits to the right of the decimal point, the last three of which are $992$. Therefore, the decimal representation of $\frac{31}{2\cdot 5^6}$ is $\boxed{0.000992}$