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

The distance of an object from its image in a plane mirror is exactly double the distance of the object from the mirror.

Thus for the man to be 5 meters away from his image he should be at a distance half of that>

Therefore required distance of man from mirror = 5/2 = 2.5 m

Original distance of man from mirror = 10 m (given)

Difference in original and required distance = 10 - 2.5 = 7.5 m.

Therefor:

The man must walk 7.5 m towards the mirror to be 5 m away from his image.