let us say she invested two amounts, "a" at 8% interest and "b" at 10%, per annum
for one year, she got $940
we dunno what "a" and "b" are, but we know, they total 10,000, or
a + b = 10,000
whatever they're
we also know, their interest yield was 940 in total
so... 8% of a, or 8/100 * a or 0.08a
added to
10% of b, or 10/100 * b or 0.10b
they add up to 940
thus 0.08a + 0.10b = 940
thus [tex]\bf \begin{cases}
a+b=10,000\implies \boxed{b}=10,000-a
\\\\
0.08a+0.10b = 940\\
--------------\\
0.08a+0.10(\boxed{10,000-a}) = 940
\end{cases}[/tex]
solve for "a", to see how much was invested at 8%
what about "b"? well, b = 10,000 - a
