From a window 10 m above the ground, the angle of elevation of the top of a lamp post across the street is 30° and the angle of depression of its foot is 60°. The height of the lamp post is:

The depression of the foot fixes the street width: d = 10·cot60° = 10/√3 m. The top of the post is then d·tan30° = (10/√3)(1/√3) = 10/3 m above the window. Adding the window's own height gives 10 + 10/3 = 40/3 ≈ 13.3 m. Reporting only the part above the window gives 10/3 m, and swapping the two angles gives a very different figure.