\[4x^{2}y^{5} = 3 \Longrightarrow \ \ x^{2}y^{5} = \frac{3}{4}:\]
\[4x^{6}y^{15} = 4\left( x^{2}y^{5} \right)^{3} = 4 \cdot \left( \frac{3}{4} \right)^{3} =\]
\[= \frac{4 \cdot 27}{4 \cdot 16} = \frac{27}{16} = 1\frac{11}{16}.\]