Center Of Mass And Collision

Bob $B$ of mass $m$ at rest is hanging vertically from the ceiling via a massless string of length 10 m , as shown in the figure. Point mass $A$ of mass $m$ travelling horizontally with speed $10 \mathrm{~ms}^{-1}$ hits bob $B$ elastically. The bob $B$ rises $h$ meter after the collision. Taking the acceleration due to gravity $g=10 \mathrm{~ms}^{-2}$ and neglecting the size of the bob, the value of $h$ is: