Three identical train cars, coupled together, are rolling east at 3.71 m/s. A fourth car travelling east at 6.74 m/s catches up with the three and couples to make a four-car train. A moment later, the train cars hit a fifth car that was at rest on the tracks, and it couples to make a five-car train. All 5 cars are identical. What is the speed of the five-car train?
m1v1+m2v2+... = m1v1'+m2v2'+...