well, there are 12 months in a year, and 9 months is not even a year, however it's 9/12 of a year.
[tex]\bf ~~~~~~ \textit{Simple Interest Earned} \\\\ I = Prt\qquad \begin{cases} I=\textit{interest earned}\\ P=\textit{original amount deposited}\to & \$800\\ r=rate\to 5\%\to \frac{5}{100}\to &0.05\\ t=years\to \frac{9}{12}&\frac{3}{4} \end{cases} \\\\\\ I=(800)(0.05)\left( \frac{3}{4} \right)\implies I=30[/tex]
