5. (5 points) Given that $a$ is a digit from 1 to 9, if the repeating decimal $0.1 \mathrm{a}=\frac{1}{\mathrm{a}}$, then $a=$
【Answer】Solution: Since $0.1 \mathrm{a}=\frac{1}{\mathrm{a}}, 0.16=\frac{1}{6}$, therefore $a=6$.

The answer is: 6.