The angle between the mirror and the beam reflected from it is 30%. What is the angle between the mirror and the incident beam?

∠φ = 30.
∠γ -?
According to the law of reflection: the angle of incidence is equal to the angle of reflection ∠α = ∠β.
The angle of incidence ∠α is the angle between the incident beam and the perpendicular drawn to the point of incidence to the reflecting surface.
The angle of reflection ∠β is the angle between the reflected beam and the perpendicular drawn to the point of incidence to the reflecting surface.
Since ∠α + ∠γ = 90 and ∠β + ∠φ = 90 and ∠α = ∠β, then ∠γ = ∠φ = 30.