Answer
$21.6^{\circ}$
Work Step by Step
We know that
$\theta_r=tan^{-1}(\frac{5.00cm}{20.0cm})=14.03^{\circ}$
Now we can find the required angle of incident as follows:
$\theta_i=sin^{-1}((\frac{n_{glass}}{n_{air}})sin\theta_r)$
We plug in the known values to obtain:
$\theta_i=sin^{-1}[(\frac{1.52}{1})sin(14.03^{\circ})]$
$\theta_i=21.6^{\circ}$
