now, there are 60 minutes in 1 hour, thus 20 minutes is just 20/60 or 1/3 of an hour.
[tex]\bf \textit{Amount for Exponential Decay using Half-Life}
\\\\
A=P\left( \frac{1}{2} \right)^{\frac{t}{h}}\qquad
\begin{cases}
A=\textit{accumulated amount}\\
P=\textit{initial amount}\to &200\\
t=\textit{elapsed time}\to &3\\
h=\textit{half-life}\to &\frac{1}{3}
\end{cases}
\\\\\\
A=200\left( \frac{1}{2} \right)^{\frac{~3~}{\frac{1}{3}}}\implies A=200\left( \frac{1}{2} \right)^{\frac{3}{1}\cdot \frac{3}{1}}\implies A=200\left( \frac{1}{2} \right)^9
\\\\\\
A=0.390625[/tex]
