Center Of Mass

Two balls \(A\) and \(B\), of masses \(M\) and \(2 M\) respectively collide each other. If the ball \(A\) moves with a speed of \(150 \mathrm{~ms}^{-1}\) and collides with ball \(B\), moving with speed \(v\) in the opposite direction. After collision if ball \(A\) comes to rest and the coefficient of restitution is 1 (one), then the speed of the ball \(B\) before it collides with ball \(A\) is