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"Veinte" is an adjective which is often translated as "twenty", and "a las tres" is a phrase which is often translated as "at three". Learn more about the difference between "veinte" and "a las tres" below.
veinte(
beyn
-
teh
)An adjective is a word that describes a noun (e.g., the big dog).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
2. (number)
b. twentieth  (in dates)
El veinte del mes que viene me voy a España.I'm going to Spain on the twentieth of next month.
a las tres(  
ah
lahs
trehs
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. at three 
Entro a trabajar a las ocho de la mañana y salgo a las tres de la tarde.I start work at eight in the morning and finish at three in the afternoon.
b. at three o'clock 
El avión salió a las tres de Toronto, así que estará a punto de llegar a Nueva York.The plane left Toronto at three o'clock so it will be about to arrive in New York.