Note that also in numbers, the word uno (one) changes to match the gender of the noun it refers to. Before a masculine noun, it becomes un, and before a feminine noun it becomes una.

Tengo treinta y un libros.

I have thirty-one books.

Hay setenta y una habitaciones en el hotel.

There are seventy-one rooms in the hotel.

 

Study the hundreds carefully too. Some of them are slightly irregular. Note that the word y is used between tens and ones, but not between hundreds and tens (or ones).

Note that hundreds also have feminine forms, which are used with feminine words:

doscientas camisetas

200 t-shirts

setecientas noventa y cuatro estrellas

794 stars