A convex lens (of focal length 20 cm) and a
concave mirror, having their principal axes
along the same lines, are kept 80 cm apart from
each other. The concave mirror is to the right
of the convex lens. When an object is kept at
a distance of 30 cm to the left of the convex
lens, its image remains at the same position
even if the concave mirror is removed. The
maximum distance of the object for which this
concave mirror, by itself would produce a
virtual image would be :-