The sun’s rays hit the earth’s surface at an angle of 40 degrees. At what angle to the horizon should

The sun’s rays hit the earth’s surface at an angle of 40 degrees. At what angle to the horizon should a flat mirror be placed so that the rays, reflected from it, go vertically?

Data: φ (the angle of incidence of the considered sun rays on the earth’s surface) = 40º.
1) The angle of installation of a flat mirror when the rays under consideration are reflected vertically upward: τ = (90 – φ) / 2 = (90 – 40) / 2 = 25º.
2) The angle of installation of a flat mirror when the rays under consideration are reflected vertically downward: τ = (90 + φ) / 2 = (90 + 40) / 2 = 65º.
Answer: A flat mirror must be placed at an angle of 25º (the rays will go vertically up) or 65º (the rays will go vertically down).