The half life of a radioactive substance is 5 years. After $x$ years a given sample of the radioactive substance gets reduced to $6.25\%$ of its initial value. The value of $x$ is $\_\_\_\_$.
The half life of a radioactive substance is 5 years. After $x$ years a given sample of the radioactive substance gets reduced to $6.25\%$ of its initial value. The value of $x$ is $\_\_\_\_$.