The number 80 (quatre-vingt) only takes an s in round numbers
☞ 80 : quatre-vingts
☞ 280 : deux cent quatre-vingts
It also takes ans when followed by a noun:
☞ Le tour du monde en quatre-vingts jours. Around the world in eighty days.
It is invariable for all other numbers
☞ 82 : quatre-vingt-deux
The number 100 cent only takes an s in round numbers
☞ 200, 300 etc. : deux cents, trois cents etc.
For all other numbers it is invariable