I apologize for the simplicity of this question, if necessary.

I understand that the sum of all 4-digit palindromes is as follows:
$$\sum\_{a=1}^{9}(\sum\_{b=0}^{9}abba),$$
which can be further expanded upon such that:

\begin{equation}
 \begin{aligned}
\sum\_{a=1}^{9}(\sum\_{b=0}^{9}abba) &= \sum\_{a=1}^{9}\sum\_{b=0}^{9}(1001a + 110b) \\
 &= \sum\_{a=1}^{9}[10(1001a) + 110\sum\_{b=0}^{9}b].
 \end{aligned}
\end{equation}
The last line of the equation is what's confusing me. Where does the $10$ come from in $10(1001a)$?   

I don't use double summation notation often, so I assume it has to do with moving the summation of $b$ inside the equation to make the triangular number formula more apparent, but WHY do we need the $10$?   

Thank you for any help.