[tex]\textit{Amount for Exponential Decay using Half-Life} \\\\ A=P\left( \frac{1}{2} \right)^{\frac{t}{h}}\qquad \begin{cases} A=\textit{current amount}\\ P=\textit{initial amount}\dotfill &1216\\ t=\textit{elapsed time}\dotfill &39\\ h=\textit{half-life}\dotfill &13 \end{cases} \\\\\\ A=1216\left( \frac{1}{2} \right)^{\frac{39}{13}}\implies A=1216\left( \frac{1}{2} \right)^3\implies A=152[/tex]
