\[\frac{5a^{5}b^{2}}{28mn^{2}} \cdot \frac{8am^{4}}{15bn^{3}} \cdot \frac{21b^{3}n^{6}}{32a^{6}m^{3}} =\]
\[= \frac{5a^{5}b^{2} \cdot 8am^{4} \cdot 21b^{3}n^{6}}{28mn^{2} \cdot 15bn^{3} \cdot 32a^{6}m^{3}} =\]
\[= \frac{b^{4}}{16n}\]