= Solution
For near-normal viewing through the flat top surface, the refracted ray appears to originate at an apparent depth $h_{\rm app}$. A ray at horizontal offset $x$ has $x=h\tan\theta_w=h_{\rm app}\tan\theta_a$, and <Snell's law> gives $n\sin\theta_w=\sin\theta_a$. In the paraxial limit,
$$
\boxed{h_{\rm app}=\frac hn=\frac{20\,\mathrm{cm}}{1.33}\simeq15.0\,\mathrm{cm}.}
$$
At an oblique viewing angle the exact ray-based apparent depth is $h\tan\theta_w/\tan\theta_a$; the simple ratio presumes the usual near-normal top view.
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