Liam from Wilbarston School sent us this
well-explained solution:
Only the middle thirds of line segments are removed, so
endpoints of line segments will not be removed.
Therefore, all I need to do is determine which are endpoints
and which are not.
$\frac{1}{3}$ is an endpoint of C2, so it is in the Cantor
set.
$\frac{3}{81}$ = $\frac{1}{27}$
$\frac{1}{27}$ is an endpoint of C4, and therefore
$\frac{3}{81}$ is in the Cantor set.
$\frac{4}{9}$ is between $\frac{1}{3}$ and $\frac{2}{3}$, thus
it is in the middle third of C1 and is removed from C2 and as such
isn't in the Cantor set.
$\frac{4}{81}$ is between $\frac{1}{27}$ and $\frac{2}{27}$
and therefore is in the middle third of C3.
It is removed from C4 and therefore does not belong.