Billy has been asked to work out the height of a tower which is on the other side of a river. He only has an inclinometer (to measure angles) and a measuring tape. He doesn't know how wide the river is.
If Billy stands directly opposite the tower, the angle of elevation is 30$^\text{o}$, and if he walks 36 metres backwards, the angle of elevation is 24$^\text{o}$, as shown in the diagram.