neverminding for a second that she's depositing 10% of 400 every month
let's say, she goes to the bank, drops 400, and goes home and sits comfy for 12 months
now, the bank every month, is giving her 10%
how much is the amount at the end of the year? or at the 12th deposit?
well, is just a matter of compound interest
[tex]\bf \qquad \textit{Compound Interest Earned Amount}
\\\\
A=P\left(1+\frac{r}{n}\right)^{nt}
\quad
\begin{cases}
A=\textit{compounded amount}\\
P=\textit{original amount deposited}\to &\$400\\
r=rate\to 10\%\to \frac{10}{100}\to &0.1\\
n=
\begin{array}{llll}
\textit{times it compounds per year}\\
\textit{is every month, thus}
\end{array}\to &12\\
t=years\to &1
\end{cases}
\\\\\\
A=400\left(1+\frac{0.1}{12}\right)^{12\cdot 1}[/tex]
