let's say she invested "a" amount at 7% and "b" amount at 9%
we know, whatever "a" and "b" are, they add up to 15000, thus
a + b = 15000
how much is 7% of a? well, (7/100) * a, or 0.07a
how much is 9% of b? well, (9/100) * b, or 0.09b
we know the sum of the interest for both is 1230, thus
0.07a + 0.09b = 1230
thus [tex]\bf \begin{cases}
a+b=15000\implies \boxed{b}=15000-a\\
0.07a+0.09b=1230\\
----------\\
0.07a+0.09\left( \boxed{15000 - a} \right) = 1230
\end{cases}[/tex]
solve for "a", to see how much was invested at 7%
what about "b"? well, b = 15000 - a
