The half life of $_{38} Sr ^{90}$ is 28 years . What is the disintegration rate of $15\;mg$ of this isotope?

Question

$\begin{array}{1 1} 7.877 \times 10^{10} Bg \\ 7.6 \times 10^{10} Bg \\ 6.877 \times 10^{10} Bg \\ 5.877 \times 10^{10} Bg \end{array}$