Answer
The index of refraction of the walls is $~~1.52$
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 index of refraction of the walls $n_2$:
$d = (\frac{n_2}{n_1})~d'$
$n_2 = \frac{n_1~d}{d'}$
$n_2 = \frac{(1.33)(4.00~mm)}{3.50~mm}$
$n_2 = 1.52$
The index of refraction of the walls is $~~1.52$
