Vectors will not follow the rules of general algebra. To determine the magnitude of a vector quantity, you have to follow the rules of vector algebra. 1. Suppose you are sitting in a moving train with a velocity of 80 m/s. And another train is coming from the opposite direction with a real velocity of 60 m/s. But when you look at that train, the train goes out of your side at a very fast speed. And then when you are asked what was the velocity of your respect other train?