Learning to count in another language isn’t always easy. Counting in French up to 69 is not too complicated, but from 70 onwards you have to do some arithmetic.
For example for 70 you have to to add 60 + 10 to get “soixante-dix”; for 80 you have to multiply 4×20 (“quatre-vingts”); and for 90 you have to multiply 4×20 and add 10 (“quatre-vingt-dix”!).
Fortunately, in other French-speaking countries you count in a more “logical” way: in Belgium you say “septante”, “quatre-vingt” and “nonante”; in Switzerland “septante”, “huitante” and “nonante”.
It is also true that the system adopted by the Romans was not that simple: for the 9 (Roman number: IX), for example, a subtraction had to be made. If you want to learn more, you will find more content at the A1 level French course. Register on Sillabi!