Answer
4.0 years
Work Step by Step
After one half-life, the radioactive isotope decays to 50% of its radioactivity. After 2 half-lives, it decays to 25% of its radioactivity and after 3 half-lives it decays to 12.5% of its radioactivity.
$\implies$ 3 half-lives=12 years
or 1 half-life= 4.0 years.
