now... we know that, whatever the earned interest was,
the quantity of "a" is bigger than "b", by 880.07 bucks,
or
a = b + 880.07
whatever "b" is, add 880.07 to be able to level with "a"
so [tex]\bf \begin{cases}
a+b=10,637\to a=\boxed{10,637-b}
\\\\
\frac{13}{100}a=\frac{6}{100}b+880.07\\
or\\
0.13a = 0.06b + 880.07\\
--------------\\
0.13(\boxed{10,637-b}) = 0.06b + 880.07
\end{cases}[/tex]
solve for "b", to see how much that was at 6%,
what about "a"? well, a = 10,637 - b
