Olympiad Geometry · Challenge problem

To measure a tower, Maya lies on level ground with her eye at ground level and a $2$ m vertical stick placed $3$ m in front of her eye, so that the top of the stick lines up exactly with the top of the tower. She then moves $20$ m straight back, away from the tower, and finds that the top of the stick lines up with the top of the tower when the stick is $4$ m in front of her eye. How tall is the tower, in metres?