A man stands 10 m in front of a large plane mirror. How far must he walk before he is 5 m away from his image?

Solution
If a person is 10 m from the mirror, then the image is formed 10 m from the mirror backwards, then the image is 20 m from the man. If the image has to be formed 5 m from the mirror, then the man is actually 5/2 i.e 2.5 m from the mirror. So to cover the full distance of 10 m, he has to cover a remaining distance of 7.5 m