[tex]\bf \qquad \textit{Amount for Exponential Decay}
\\\\
A=P(1 - r)^t\qquad
\begin{cases}
A=\textit{accumulated amount}\\
P=\textit{initial amount}\\
r=rate\to r\%\to \frac{r}{100}\\
t=\textit{elapsed time}\\
\end{cases}
\\\\\\
\stackrel{A}{B(t)}=\stackrel{P}{520}(\stackrel{1-r}{0.4})^{1.8t}+11[/tex]
the tell-tale factor on this exponential equations is the "growth factor", namely the parenthesized value.
if the value is greater than 1, is growth, if the value is less than 1, is decay.
notice in this case is 0.4, so the "growth factor" is less than 1.
