Night after night, Lucy is tormented by nocturnal wailing from the house next door. One day she seizes a revolver, stalks to the neighbor's window with a crazed look in her eye, takes aim, and fires a 15 g bullet into her target: a 1.8 kg saxophone that rests on a frictionless surface. The bullet passes right through and emerges on the other side with a speed of 100 m/s, and the saxophone is given a speed of 4 m/s.
Find the initial speed of the bullet.