 So Carl chucked the ball relative to himself at 10 meters per second, but Bob sees Carl already moving at 90 meters per second. So in Bob's reference frame the ball is moving at 100 meters per second. Alice is standing on the train and the train is moving at 90 meters per second. So Bob will see Alice as moving at 90 meters per second. So finally let's ask how fast does Bob see the ball collide with Alice's hands?