\[\frac{5 - \sqrt{10}}{\sqrt{10} - 2} = \frac{\sqrt{5}\left( \sqrt{5} - \sqrt{2} \right)}{\sqrt{2}\left( \sqrt{5} - \sqrt{2} \right)} =\]
\[= \frac{\sqrt{5}}{\sqrt{2}} = \frac{\sqrt{10}}{2}.\]