[tex]I = Prt\qquad \begin{cases} I=\textit{interest earned}\dotfill & \$1800\\ P=\textit{original amount deposited}\dotfill & \$5000\\ r=rate\to r\%\to \frac{r}{100}\\ t=years\dotfill &9 \end{cases} \\\\\\ 1800=5000(\frac{r}{100})(9)\implies \cfrac{1800}{5000(9)}=\cfrac{r}{100}\implies \cfrac{1}{25}=\cfrac{r}{100} \\\\\\ 100=25r\implies \cfrac{100}{25}=r\implies \stackrel{\%}{4}=r[/tex]
