Question

The half-life of a radioactive isotope is the time it takes for a quantity of the Isotope to be reduced to half Its Initial mass. Starting with 165 grams of a
radioactive isotope, how much will be left after 3 half-lives??

Answer 1

$\stackrel{\textit{half of 165}}{165\cdot \cfrac{1}{2}}~\hfill \stackrel{\textit{half of half of 165}}{165\cdot \cfrac{1}{2}\cdot \cfrac{1}{2}}~\hfill \stackrel{\textit{half of half of half of 165}}{165\cdot \cfrac{1}{2}\cdot \cfrac{1}{2}\cdot \cfrac{1}{2}} \\\\\\ 165\cdot \cfrac{1}{8}\implies \cfrac{165}{8}\implies 20.625~grams$