An object and a concave mirror of focal length $$f = 10$$ cm both move along the principal axis of the mirror with constant speeds. The object moves with speed $$V_0 = 15$$ cm s$$^{-1}$$ towards the mirror with respect to a laboratory frame. The distance between the object and the mirror at a given moment is denoted by $$u$$. When $$u = 30$$ cm, the speed of the mirror $$V_m$$ is such that the image is instantaneously at rest with respect to the laboratory frame, and the object forms a real image. The magnitude of $$V_m$$ is _______ cm s$$^{-1}$$.