\[\frac{2a^{3}}{25b^{3}} \cdot \frac{10b^{2}}{3c^{4}}\ :\frac{4a^{2}}{15bc^{3}} =\]
\[= \frac{2 \cdot 10 \cdot 15a^{3}b^{3}c^{3}}{25 \cdot 3 \cdot 4a^{2}bc^{4}} = \frac{a}{c}\]