The shadow of a vertical tower on level ground is 15 m long when the sun's altitude is 60°. When the altitude falls to 30°, the length of the shadow becomes:

The tower's height is h = 15·tan60° = 15√3 m. At 30° the shadow is h·cot30° = 15√3 × √3 = 45 m. (Only doubling the shadow because the angle halved gives the wrong 30 m; 15√3 ≈ 25.98 m is the tower's height.)