Hi Justine,
If you stand 1 meter, for example, in front of a plane mirror, the image that you see in the mirror looks like it's 1 meter behind the mirror, for a total of 2 meters.
So if someone has the ability to see clearly at 300 cm, then if they were in front of a plane mirror they would have to stand 150 cm infront of the mirror to see their image clearly, as their image would appear to be 300 cm away.