posted by Susy .
A light bulb is placed 16.6 cm in front of a concave mirror. When the concave mirror is replaced by a plane mirror in the same location, the bulb's image moves 5.40 cm closer to the mirror. Calculate the focal length of the concave mirror.
i know that for plane mirrors distance of object=distance of image so i should just add 5.40 cm to 16.6 for di of the concave mirror, but it keeps saying my answer is wrong