two cars on frictionless surface carry two persons.

two cars on frictionless surface carry two persons.one person in car A throws a ball of mass 1kg with speed of 10m/s to the person in car B and one in car B throws a ball of mass m=1kg with speed 20m/s back to the person in car A. if the mass of person and car same and 99kg and each ball has mass 1kg what is the relative velocity of car A with respect to car B?at the beginning both car remains motionless