Find out the third proportional to $16$ and $36$.

Solve for the third proportional.

Given the third be proportional to$16$ and $36$ be $x$.

$\Rightarrow$ $16:36::36:x$

$$\begin{gathered} \Rightarrow \frac{16}{36}=\frac{36}{x} \\ \Rightarrow 16\times x=36\times 36 \\ \Rightarrow x=\frac{36\times 36}{16} \\ \Rightarrow x=81 \end{gathered}$$

Hence, the third proportion of $16$ and $36$ is $81$.