\[\frac{m - 8}{5m}\ :\frac{m^{2} - 64}{15m^{2}} =\]
\[= \frac{m - 8}{5m} \cdot \frac{15m^{2}}{(m - 8)(m + 8)} =\]
\[= \frac{3m}{m + 8}.\]