The distance between an image and a plane mirror is equal to the distance between the object and the mirror. So if the object is 15m in front of the mirror, the image will be 15m behind the mirror. This can be explained by the law of reflection, which states that the angle of incidence (the angle at which the light strikes the mirror) is equal to the angle of reflection (the angle at which the light bounces off the mirror). In other words, the light rays bounce off the mirror in a straight line, so the image is formed at the same distance behind the mirror as the object is in front of it.