well, bearing in mind that, a year has 12 months, so 18 months is really just 18/12 of a year, or 3/2 a year, then
[tex]\bf \qquad \textit{Simple Interest Earned Amount}\\\\
A=P(1+rt)\qquad
\begin{cases}
A=\textit{accumulated amount}\\
P=\textit{original amount deposited}\to& \$900\\
r=rate\to 4\%\to \frac{4}{100}\to &0.04\\
t=years\to \frac{18}{12}\to &\frac{3}{2}
\end{cases}
\\\\\\
A=900\left( 1+0.04\cdot \frac{3}{2} \right)[/tex]
