well, there are 12 months in a year, so 18 months will then be 18/12 of a year
[tex]\bf ~~~~~~ \textit{Simple Interest Earned}\\\\
I = Prt\qquad
\begin{cases}
I=\textit{interest earned}\to &\$54\\
P=\textit{original amount deposited}\to& \$900\\
r=rate\to r\%\to \frac{r}{100}\\
t=years\to \frac{18}{12}\to &\frac{3}{2}
\end{cases}
\\\\\\
54=(900)(r)\left( \frac{3}{2} \right)\implies \cfrac{54}{(900)\left( \frac{3}{2} \right)}=r\implies \cfrac{1}{25}=r\implies 0.04=r
\\\\\\
r\%=0.04\cdot 100\implies r=\stackrel{\%}{4}[/tex]
