[tex]\bf \textit{Periodic Exponential Decay}\\\\
A=I(1 - r)^{\frac{t}{p}}\qquad
\begin{cases}
A=\textit{accumulated amount}\\
I=\textit{initial amount}\\
r=rate\to r\%\to \frac{r}{100}\\
t=\textit{elapsed time}\\
p=period
\end{cases}[/tex]
we know, the first year she sold 400 units, thus year 0, 400 units, t = 0, A = 400.
[tex]\bf \textit{Periodic Exponential Decay}\\\\
A=I(1 - r)^{\frac{t}{p}}\qquad
\begin{cases}
A=\textit{accumulated amount}\to &400\\
I=\textit{initial amount}\\
r=rate\to r\%\to \frac{r}{100}\\
t=\textit{elapsed time}\to &0\\
p=period
\end{cases}
\\\\\\
400=I(1-r)^{\frac{0}{p}}\implies 400=I\cdot 1\implies 400=I[/tex]
therefore then
[tex]\bf \textit{Periodic Exponential Decay}\\\\
A=I(1 - r)^{\frac{t}{p}}\qquad
\begin{cases}
A=\textit{accumulated amount}\\
I=\textit{initial amount}\to &400\\
r=rate\to 20\%\to \frac{20}{100}\to &0.20\\
t=\textit{elapsed time}\\
p=period\to &2
\end{cases}
\\\\\\
A=400(1-0.2)^{\frac{t}{2}}[/tex]
now, how many units will it had decreased by the 9th year? t = 9
[tex]\bf A=400(1-0.2)^{\frac{9}{2}}[/tex]
