[tex]\bf \qquad \textit{Simple Interest Earned Amount}\\\\
A=P(1+rt)\qquad
\begin{cases}
A=\textit{compounded amount}\to &10,320\\
P=\textit{original amount }\to& \$8000\\
r=rate\\
t=years\to &4
\end{cases}[/tex]
plug in the values, and solve for "r"
you'll get a decimal amount, so multiply it by 100, to get the percentage figure for the rate used
