The Physics Problem Solver/Mechanics/Work and Energy

A car of mass 1000 kg is parked on a level road with its handbrake on. Another car of mass 1500kg travelling at 10ms-1 collides with the back of the stationary car. The two cars move together after collision in the same straight line. They travel 25m before finally coming to rest. Find the average frictional force exerted on the cars as they come to rest.