353
Each article you create can have multiple pages eg. positrons >>> 1st page (easy) 2nd page (medium) 3rd page (hard).
Not sure if that will suit you, just an idea ;o)
Edit: I just did a quick test it looks like when you make an article with multiple pages, you can add the pages to multiple categories, it was just a quick test, it may be worth a try for what you want to achieve ;o)
Oh bugger not sure an individual page within an article can be added to a separate categoory, anyway see how you go.