Does the un/una variation apply everywhere

Question

Hello, excellent lesson. We know that "un/una" changes with gender. Is this true of every number that ends with "un/una"?

Ex:

Vientiún estudiantes, veintiuna camas

Ciento un aguacates, ciento una botellas

Thanks.

Accepted Answer

Hola Marcos
Yes, exactly! 
When a number ending in "one" (un) precedes a masculine plural noun you need "un" and if it is a feminine plural noun you need "una".

treinta y un niños
treinta y una niñas
cincuenta y un pájaros
cincuenta y una vacas

When it's followed by "mil" (thousand) it admits both with feminine plural nouns though:

ciento veintiún mil vacas
ciento veintiuna mil vacas
121,000 cows

Saludos
Inma