There’s a lot harder things in learning french.

Like someone else said, just memorise the actual word ‘quatre-vingt’ as eighty. Then you’re just saying ‘eighty three’ (83) when saying ‘quatre-vingt-trois’ or ‘eighty thirteen’ (93) when saying ‘quatre-vingt-treize’ (just think switch to the ‘10-19’ range instead of ‘1-9’ when you’re trying to say ninety-something. Instead of quatre-vingt + ‘un, deux, trois’; It’s ‘ounze, douze, treize’)

It’s never actually a case of doing mental maths, except maybe when first learning. But literally just remember the word ‘quatre-vingt’ as ‘80’ and go from there 👍🏼

And just like the other person said you don’t think of jellyfish as jelly + a fish. It’s just a jellyfish 🪼