4. Two guns are mounted a distance of 40 Ins (light-nanoseconds) apart on the embankment beside a railroad track. The barrels of the guns project towards the track so that they almost brush a speeding express train as it passes by. The train moves with a speed of v = 3/5 c with respect to the ground. Suppose the two guns fire simultaneously (as measured in the ground frame), leaving two bullet holes in the train. Find the distance between the bullet holes measured by a passenger on the train. Justify your answer from the point of view of an observer on the ground and an observer on the train.