An object and a concave mirror of focal length $f=10 \mathrm{~cm}$ both move along the principal axis of the mirror with constant speeds. The object moves with speed $\mathrm{V}_0=15 \mathrm{~cm} \mathrm{~s}^{-1}$ 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 $\mathrm{u}=30 \mathrm{~cm}$, the speed of the mirror $\mathrm{X}_{\infty}$ 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 $\mathrm{X}_{\infty}$ is $\_\_\_\_$ $\mathrm{cm} \mathrm{s}^{-1}$.