Final answer:
A nearsighted man, who can see clearly up to 20 cm, must stand about 10 cm away from a mirror to see his reflection clearly while shaving.
Step-by-step explanation:
The question is about a nearsighted man who cannot see objects clearly beyond 20 cm. When it comes to seeing his reflection clearly in the mirror while shaving, he just has to stand close enough so that the image of his face in the mirror falls within that 20 cm range where his vision remains clear.
Since the mirror reflects light back at the same angle, for the man to see his reflection clearly, the distance from his eyes to the mirror should be half of the maximum distance he can see clearly, which is 20 cm. Therefore, he must stand approximately 10 cm away from the mirror.
So that the total distance from his eyes to his reflected image (which is the distance to the mirror plus the distance from the mirror to his face) is 20 cm, falling within his clear vision range.