16. Evaluate
$$
\frac{1}{\log _{2} 12 \sqrt{5}}+\frac{1}{\log _{3} 12 \sqrt{5}}+\frac{1}{\log _{4} 12 \sqrt{5}}+\frac{1}{\log _{5} 12 \sqrt{5}}+\frac{1}{\log _{6} 12 \sqrt{5}} .
$$
16. Ans: 2
$$
\begin{aligned}
& \frac{1}{\log _{2} 12 \sqrt{5}}+\frac{1}{\log _{3} 12 \sqrt{5}}+\frac{1}{\log _{4} 12 \sqrt{5}}+\frac{1}{\log _{5} 12 \sqrt{5}}+\frac{1}{\log _{6} 12 \sqrt{5}} \\
= & \log _{12 \sqrt{5}} 2+\log _{12 \sqrt{5}} 3+\log _{12 \sqrt{5}} 4+\log _{12 \sqrt{5}} 5+\log _{12 \sqrt{5}} 6 \\
= & \log _{12 \sqrt{5}} 2 \times 3 \times 4 \times 5 \times 6 \\
= & \log _{12 \sqrt{5}} 720 \\
= & 2, \text { since }(12 \sqrt{5})^{2}=720 .
\end{aligned}
$$