\[3ab^{4} = 5 \Longrightarrow \ \ ab^{4} = \frac{5}{3}\]
\[6a^{3}b^{12} = 6 \cdot \left( ab^{4} \right)^{3} = \frac{6 \cdot 125}{27} =\]
\[= \frac{2 \cdot 125}{9} = \frac{250}{9} = 27\frac{7}{9}\]