Suppose in this direction is a positive direction. Now suppose another car is also traveling 60 mph, but in the other direction, so it's negative 60, where 60 is our magnitude and direction is our negative sign. So if someone asks you what is the average of the first two cars, from a physics perspective you will say 60 mph. Because if one car is going 60 and the other car is going 60, then the average must be 60. Well, if you use the formula to find the average from a mathematics point of view, you will see that it's 60 plus -60, gives you zero divided by two, which is zero. So sometimes from a mathematics point of view, taking the average in the same way that you did here doesn't make sense.