Answer
$20^\circ$
Work Step by Step
From the geometry of the figure below, we can see that angle $\phi$ is 20$^\circ$.
The main idea here is the two angles of 40$^\circ$ of the two parallel lines that are parallel to the bisector of angle $80^\circ$. And hence, the two 40$^\circ$s angles appear.
Also, we know that the angle of the incident ray relative to the normal is equal to the reflected ray angle relative to the same normal as well.
Therefore, we got
$$\phi=\color{red}{\bf20^\circ}$$
