keeping in mind that a year has 12 months, so that means that 18 months is really 18/12 of a year.
[tex]~~~~~~ \textit{Simple Interest Earned} \\\\ I = Prt\qquad \begin{cases} I=\textit{interest earned}\\ P=\textit{original amount deposited}\dotfill & \$1200\\ r=rate\to 10.5\%\to \frac{10.5}{100}\dotfill &0.105\\ t=years\to \frac{18}{12}\dotfill &\frac{3}{2} \end{cases} \\\\\\ I = (1200)(0.105)(\frac{3}{2})\implies 189[/tex]
