I think the word you were looking for is "principles", unless you are very friendly with your teachers.
How to think of this? Take a scene with a lot of "stuff" in it, all pretty much the same and all illuminated equally. Then, for a constant ISO and shutter speed, the f-number doesn't have to change to get the correct exposure whether you have a 24mm lens or a 200mm lens in use.
The trick is that the scene being photographed is composed of extended objects. But try using the same exposure, shutter speed and f-number when photographing a star and you will see a big difference between the 24mm and 200mm lenses. For a point source, in an ideal case, all the light is being concentrated on a single pixel no matter what the focal length of the lens. What matters is the diameter of the entrance pupil and you can do the math for yourself.
Hope that way of analysing the question makes sense, helps you finish off the analysis yourself and will be a useful tool in your arsenal.