An object and a concave mirror of focal length f=10 cm both move along the principal axis with constant speeds. The object moves with speed V₀=15 cm s⁻¹ towards the mirror in the laboratory frame. Let u be their separation. When u=30 cm, the mirror speed Vₘ is such that the real image is instantaneously at rest in the laboratory frame. The magnitude of Vₘ is _____ cm s⁻¹.