keeping in mind that 6 months is not even a year, since there are 12 months in a year, then 6 months is really 6/12 years, and 9 months is 9/12 years, then
[tex]\bf ~~~~~~ \textit{Simple Interest Earned on 9 months}\\\\
I = Prt\qquad
\begin{cases}
I=\textit{interest earned}\\
P=\textit{original amount deposited}\to& \$900\\
r=rate\to 2.9\%\to \frac{2.9}{100}\to &0.029\\
t=years\to \frac{9}{12}\to &\frac{3}{4}
\end{cases}
\\\\\\
I=900(0.029)\left( \frac{3}{4} \right)\\\\
-------------------------------[/tex]
[tex]\bf ~~~~~~ \textit{Simple Interest Earned on 6 months}\\\\
I = Prt\qquad
\begin{cases}
I=\textit{interest earned}\\
P=\textit{original amount deposited}\to& \$900\\
r=rate\to 2.4\%\to \frac{2.4}{100}\to &0.024\\
t=years\to \frac{6}{12}\to &\frac{1}{2}
\end{cases}
\\\\\\
I=900(0.024)\left( \frac{1}{2}\right)[/tex]
check their difference, that much.
