Numbers can be tricky and incosistent in French.
Numbers that imply multiplication tend to take an 's' to indicate a plural but in the case of quatre-vingts it loses its 's' before another numeral:
80 quatre-vingts, but 85 quatre-vingt-cinq
It is a similar case for hundreds where cent takes an 's' for round hundreds but loses it before another number
200 deux cents, but 210 deux cent dix
Thousands NEVER takes a plural
1,000 mille, 5,000 cinq millle, 10,100, dix mille cent
Million, milliard, and billion keep their 's' in all circumstances.
2,000,000 deux millions, 2,000,010 deux millions dix
2,000,000,000 deux milliards, 2,000,000,010 deux milliards dix
2,000,000,000,000 deux billions 2,000,000,000,010 deux billions dix
This should explain why cents is plural in mille neuf cents/dix-neuf cents but not anywhere else
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