In this lesson you learn to count from 16 to 29 in Spanish. You'll see that from 16 onward there is a regularity in counting. From 30 onward, slightly different rules apply, which luckily also follow a regular pattern.

0 - cero	10 - diez	20 - veinte
1 - uno	11 - once	21 - veintiuno
2 - dos	12 - doce	22 - veintidós
3 - tres	13 - trece	23 - veintitrés
4 - cuatro	14 - catorce	24 - veinticuatro
5 - cinco	15 - quince	25 - veinticinco
6 - seis	16 - dieciséis	26 - veintiséis
7 - siete	17 - diecisiete	27 - veintisiete
8 - ocho	18 - dieciocho	28 - veintiocho
9 - nueve	19 - diecinueve	29 - veintinueve

You have already learned that one before a masculine noun becomes un and before a feminine noun becomese una. The same principle applies to numbers that end in uno.

the twenty-one books	los veintiún libros
the twenty-one houses	las veintiunas casas