A cat lies on top of a low wall and looks up towards the birds in the top of a tree. There is a bush between the cat and the tree, and the cat can only just see over the top of the bush.
The bush is a horizontal distance of H metres from the wall, and the tree is a horizontal distance of H metres from the bush.
The height of the bush is 4 m. The height of the tree is 7 m.
How high is the wall?
This problem is adapted from the World Mathematics Championships