The relationship between the two unknowns is that they total \($12,500\text{.}\) When a total is involved, a common technique used to avoid two variables is to represent the second unknown as the difference of the total and the first unknown.
Let \(m\) represent the amount invested in the mutual fund.
Since the two unknowns total \($12,500\text{,}\) we let \(12,500m\) represent the remaining amount invested in the CD.
Interest earned in the mutual fund 
\begin{align*}
I\amp = prt\\
\amp = m\cdot 0.07\cdot 1\\
\amp =0.07m
\end{align*}

Interest earned in the CD 
\begin{align*}
I\amp = prt\\
\amp = (12,500m)\cdot 0.045\cdot 1\\
\amp = 0.045(12,500m)
\end{align*}

Total Interest 
\($670\) 
The total interest is the sum of the interest earned from each account.
\begin{equation*}
\begin{aligned}
\text{mutual fund interest}+\text{CD interest} \amp = \text{total interest}\\
0.07m +0.045(12,500m)\amp = 670
\end{aligned}
\end{equation*}
This equation models the problem with one variable. Solve for \(m\text{.}\)
\begin{equation*}
\begin{aligned}
0.07m+0.045(12,500m)\amp = 670\\
0.07m+562.50.045m\amp = 670\\
0.025m+562.5\amp = 670\\
0.025m\amp = 107.5\\
m\amp = \frac{107.5}{0.025}\\
m\amp =4,300
\end{aligned}
\end{equation*}
Use \(12,500m\) to find the amount in the CD.
\begin{equation*}
12,500m=12,500\alert{4300}=8,200
\end{equation*}
Stephanie invested \($4,300\) at \(7\)% in a mutual fund and \($8,200\) at \(4.5\)% in a CD.