\[\frac{m^{2} - 10mn + 25n^{2}}{12m^{3}n^{2}}\ :\frac{m - 5n}{6mn} =\]
\[= \frac{(m - 5n)^{2} \cdot 6mn}{12m^{3}n^{2}(m - 5n)} = \frac{m - 5n}{2m^{2}n}.\]