Une cinquantaine de personnes, c'est environ cinquante personnes. En fait, il y a peut-être 45 ou 52 personnes. Ajouter "aine" (comme dans dizaine, douzaine, vingtaine, etc.) permet de donner un chiffre approximatif.

Bonjour, 1. Cinquante is a precise number. =50 . It can be used to define the age of someone or to count how many people, cars, animals, things you have. Ex: Mon père à cinquante ans. or Il y a cinquante élèves dans ma classe. 2. Cinquantaire is an approximative number. It means that you don't know exactely how many people, cars, ... you have. You only know it's about (more or less) 50. It's also can be used for people when you don't know exactely how old are they. Hope it will help :)