18.1.2 un, uno, una

Numerals that end on one (twenty-one, thirty-one, hundred and one) have to be changed in accordance with the gender and number of the object they are referring to.

Further, note that uno in adjectival use before masculine nouns changes into un. (un hombre; but quiero uno que no gaste tanta gasolina)

  Examples un, uno, una

veintiún coches = 21 cars
veintiuna casas = 21 houses
ciento treinta y una mujeres = 131 women
treinta y un coches = 31 cars
treinta y una casas = 31 houses
ciento treinta y un hombres = 131 men

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