A lamp is fixed at the top of a post 6 m high. A man 1.8 m tall walks away from the foot of the post along level ground. When he is 5.6 m from the foot of the post, how long is his shadow?

Let the shadow have length s, so the shadow tip is 5.6 + s from the foot of the post. The man and his shadow form a triangle similar to the one formed by the post and the whole distance, so 1.8/s = 6/(5.6 + s). Cross-multiplying, 1.8(5.6 + s) = 6s, that is 10.08 + 1.8s = 6s, so 4.2s = 10.08 and s = 2.4 m. The value 8 m is the distance from the post to the shadow tip, not the shadow itself.