People sometimes say אחוז and sometimes say אחוזים, what is the difference?
When the number is lower than 10 you have to use the plural - "אחוזים", when it's higher you can either use the singular אחוז and or plural אחוזים. either is fine.
It's the same for almost all the nouns.
What is the etymology of אחוז?
Why can't we use the term "mean" instead of average?
You definitely should be able to say mean instead of average. I cannot even begin to imagine why anyone would downvote Phmagnabosco's question, but evidently someone did
So הוא copula here because it is specific.
Ha-memutsa hu esrim akhuz.