the amounts are "a" on the CD at 4% and "b" on the bonds at 7%
now, she had 12,000, so we know whatever "a" and "b" are, they
a + b = 12000
now... what's 4% of a? well, (4/100) * a, or 0.04a
what's 7% of b? well (7/100) * b or 0.07b
we know the total interest, that 4% and 7% amounts, add up to 720, thus
0.04a + 0.07b = 720
thus [tex]\bf \begin{cases}
a+b=12000\implies \boxed{b}=12000-a\\
0.04a+0.07b=720\\
----------\\
0.04a+\left( \boxed{12000-a} \right)=720
\end{cases}[/tex]
solve for "a", to see how much was invested in CD
what about "b"? well b = 12000 - a
