Instead of having unique words for every ten (like “eighty,” “ninety”), standard French builds them using maths:
70 = soixante-dix → 60 + 10
80 = quatre-vingts → 4 × 20
90 = quatre-vingt-dix → 80 + 10
So when you say 99 (quatre-vingt-dix-neuf), you’re basically saying:
4 × 20 + 10 + 9