he invested "a" at 3% and "b" at 5%
now, he invested 100 more in "b", so, whatever "a" may be, b = a + 100
now, how much is the interest yield of each? well
how much is 3% of a? well, (3/100) * a, or 0.03a
and 5% of b? well (5/100) * b, or 0.05b
now, we know the total yield was 53 bucks, thus 0.03a + 0.05b = 53
thus [tex]\bf \begin{cases}
\boxed{b}=a+100\\
0.03a+0.05b=53\\
----------\\
0.03a+0.05\left( \boxed{a+100} \right)=53
\end{cases}[/tex]
solve for "a", to see how much was invested at 3%
what about "b"? well, b = a + 100
