he invested "a" at 9% and "b" at 10%
now, we know he's investing 20,000, so whatever "a" and "b" are, they sum up to 20000
a + b = 20000
how much is 9% of a? well, (9/100) * a, or 0.09a
how much is 10% of b? well, (10/100) * b or 0.10b
we know, his annual interest returns are 1980 for both, so
0.09a + 0.1b = 1980
[tex]\bf \begin{cases}
a+b=20000\implies \boxed{b}=20000-a\\
0.09a + 0.1b = 1980\\
----------\\
0.09a + 0.1\left( \boxed{20000-a}\right) = 1980
\end{cases}[/tex]
solve for "a", to see how much he invested at 9%
what about "b"? well, b = 20000 - a
