# What fraction of a radioactive sample remains after three half-lives?

What is $\frac{1}{2}$ $\times$ $\frac{1}{2}$ $\times$ $\frac{1}{2}$?
Three half lives corresponds to ${\left(\frac{1}{2}\right)}^{3}$. So a $\frac{1}{8}$ quantity of the original isotope is retained.