Collisions (Elastic)

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