The twenties are written as one word — veinte fused with the unit:

21 veintiuno · 22 veintidós · 23 veintitrés · 24 veinticuatro · 25 veinticinco · 26 veintiséis · 27 veintisiete · 28 veintiocho · 29 veintinueve

Watch the accents on veintidós, veintitrés and veintiséis.

From thirty on, it's the ten, then y (and), then the unit — as separate words:

Number	Spanish	Number	Spanish
30	treinta	70	setenta
40	cuarenta	80	ochenta
50	cincuenta	90	noventa
60	sesenta	100	cien

• 31 treinta y uno · 45 cuarenta y cinco · 99 noventa y nueve

100 is cien on its own or before a noun — cien euros. It only becomes ciento when more follows: 101 ciento uno, 120 ciento veinte.