\[\frac{3x^{2}}{16 - y^{2}}\ :\frac{15x^{5}}{4 + y} =\]
\[= \frac{3x^{2}}{(4 - y)(4 + y)} \cdot \frac{4 + y}{15x^{5}} =\]
\[= \frac{1}{5x^{3}(4 - y)}.\]