An 80-lb N boy sitting in a stationary 20-lb wagon wants to simulate rocket propulsion by throwing bricks out of the wagon. Neglect horizontal forces on the wagon's wheels. If the boy has three bricks weighing 10 lb each and throws them with a horizontal velocity of 10 ft/s relative to the wagon , determine the velocity he attains (a) if he throws the bricks one at a time and (b) if he throws them all at once.