Standing 100 m from a building when the man turns his head upwards at an angle equal to 20 degrees to the horizontal he can see the top of the building.
Ignore the height of the man. There is a right triangle formed by the point at the base of the building, the point where the man is standing and the top of the building.
tan 20 = H/100 where H is the height of the building.
=> H = 100*tan 20 = 36.39
The height of the building is approximately 36.39 m