help plsn i5 might look like no latex but it has latex. Thank you!

Function  is defined on positive integers as follows:
$C(n) = \begin{cases} \dfrac n 2 & \text{if $n$ is even}, \\ 3n+1 & \text{if $n$ is odd}. \end{cases}$

Final all such that C^3(n)=16

The thing is that I go to 2 of the answers. being 128, and 20 but i cant find the others.