Answer
Ray $b$ emerges at an angle of $20^{\circ}$
Work Step by Step
Let $\theta_f$ be the angle at which ray $b$ emerges.
According to Snell's law:
$n_{air}~sin~\theta_b = n_1~sin~\theta_1 = n_2~sin~\theta_2 =... = n_{air}~sin~\theta_f$
$n_{air}~sin~\theta_b = n_{air}~sin~\theta_f$
$sin~\theta_b = sin~\theta_f$
$\theta_f = \theta_b$
$\theta_f = 20^{\circ}$
Ray $b$ emerges at an angle of $20^{\circ}$
