A vertical pole 6 m tall casts a shadow 4 m long on level ground. At the same time of day a tower nearby casts a shadow 28 m long. How tall is the tower?

The sun's rays arrive at the same angle, so the pole with its shadow and the tower with its shadow form similar right triangles: height/shadow is the same for both. That ratio is 6/4 = 1.5, so the tower's height = $1.5\times 28=42$ m. Turning the ratio upside down gives $28\times 4/6$ ≈ 18.7 m, the classic slip.