If a 1.75 m tall man is looking at a plane mirror from an eye which is 15 cm from the top of their head, what is the minimum length that the mirror must be so that he can just barely see his whole body in the mirror? How far up from the ground must the mirror be placed?

1/(object distance) + 1/(image distance) = 1/(focal length)

Magnification = image height/(object height) = -image distance/(object distance)