she invested "a" at 4% and "b" at 7%
now, how much is 4% of a? well 4/100 * a or 0.04a
how much is 7% of b? well 7/100 * b, or 0.07b
now, we know their interest yield was 480, thus
0.04a + 0.07b = 480
we also know, she had 8000 to invest and split it in two investments, so
a + b = 8000
now [tex]\bf \begin{cases}
a+b=8000\implies \boxed{b}=8000-a\\
0.04a+0.07b=480\\
----------\\
0.04a+0.07\left( \boxed{8000-a} \right)=480
\end{cases}[/tex]
solve for "a" to see how much she invested at 4%
what about "b"? well b = 8000 - a
