I've noticed that when driving down a road, cars passing in the opposite direction appear to be going much faster than if viewed from a stationary position. I asked my friend how you could calculate how fast the other car appeared to be going and he said you just add your speed and the other car's speed. I thought about this, and then had a question: if one object was going 75% the speed of light and an object going in the other direction was going 75% the speed of light as well, the apparent speed of the other object from the view of the first object would be 150% the speed of light. Since I know this is not possible, something has to change, but my friend is certain about his answer. What is the change?