This thought experiment involves two people named Person-A and Person-B. Person-A fires a gun from a car which is traveling at 60mph. Imagine that the bullet travels at a constant speed of 300mph for some time, assuming that the air friction is ignored. Person-B is stationary observing the whole "shooting from the car" action. From the point of view of Person-B, the bullet is traveling at 360mph, and from Person-A it is traveling at 300mph. Is the above assumption correct? What mechanism is behind above situation?