\[\ \frac{\sqrt{15} - 5}{3 - \sqrt{15}} = \frac{\sqrt{5}\left( \sqrt{3} - \sqrt{5} \right)}{\sqrt{3}\left( \sqrt{3} - \sqrt{5} \right)} =\]
\[= \frac{\sqrt{5}}{\sqrt{3}} = \frac{\sqrt{15}}{3}\]