Question

Sodium-24 has a half-life of 15 hours.

Answer 1

$\bf \textit{Amount for Exponential Decay using Half-Life}\\\\ A=I\left( \frac{1}{2} \right)^{\frac{t}{h}}\qquad \begin{cases} A=\textit{accumulated amount}\\ I=\textit{initial amount}\to &900\\ t=\textit{elapsed time}\to &90\\ h=\textit{half-life}\to &15 \end{cases} \\\\\\ A=900\left( \frac{1}{2} \right)^{\frac{90}{15}}\implies A=900\left( \frac{1}{2} \right)^6$