I assume, ordinary, means "simple interest"
well, i = prt, "i" is how much she'll be paying
now, from september 20th, to october 20th, to november 20th, to december 20th, to january 20th, is only hmm let's add those days
30+31+30+31 so hmmm 122 days
now, the year has 365 days, 122/365 years then
[tex]\bf \qquad \textit{Simple Interest Earned}\\\\
I = Prt\qquad
\begin{cases}
I=\textit{interest earned}\\
P=\textit{original amount deposited}\to& \$7000\\
r=rate\to 8\%\to \frac{8}{100}\to &0.08\\
t=years\to &\frac{122}{365}
\end{cases}[/tex]
