Simplify : $ \large\frac{25 \times a^{-4}}{5^{-3} \times 10 \times a^{-6}}$ where $a \neq 10$

Question

$\begin{array}{1 1} \frac{625}{2} a^2 \\ \large\frac{a^2}{2} \\ \large\frac{5^4}{2} \\ 625\;a^2 \end{array} $