Answer
$d = 22.6~cm$
Work Step by Step
We can use this formula:
$d = (\frac{n_2}{n_1})~d'$
where $d$ is the actual distance and $d'$ is the apparent distance.
We can find the actual distance:
$d = (\frac{n_2}{n_1})~d'$
$d = (\frac{1}{1.33})~(30~cm)$
$d = 22.6~cm$
