\[- 1\frac{2}{3}m^{3}n^{8} \cdot \left( - \frac{1}{5}mn^{4} \right)^{2} =\]
\[= - \frac{5}{3}m^{3}n^{8} \cdot \frac{1}{25}m^{2}n^{8} =\]
\[= - \frac{1}{15}m^{5}n^{16}\]