Question 11.3: Strontium-90 is a nuclide found in radioactive fallout from ......

If 15/16 of the sample has decayed, then 1/16 of the sample remains undecayed. In terms of
1/2^{n}, 1/16 is equal to 1/2^{4}; that is,
\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}=\frac{1}{2^{4}}=\frac{1}{16}
Thus 4 half-lives have elapsed in reducing the amount of strontium-90 to 1/16 of its original amount.
The half-life of strontium-90 is 28 years, so the total time elapsed will be
4  \cancel{half-lives}\times (\frac{28.0  years}{1  \cancel{half-life}}) = 112 years