A firefighter is standing on a 40.0-kg flatbed cart that is initially at rest on a frictionless surface. The firefighter has a mass of 60.0 kg. He throws a 7.0-kg fire extinguisher horizontally with a speed of 8.0 m/s to a fellow firefighter standing nearby. Calculate the velocity of the cart immediately after the extinguisher is thrown, assuming it was initially at rest.