If unos/unas is used before a number, it usually means "about", "some" or "approximately". Like adjectives, the article must agree with the noun in number and gender.
La película dura unos ochenta minutos.The film lasts about eighty minutes.
El taxi tardó unos veinte minutos.The taxi took about twenty minutes.
Tengo unos cincuenta euros en la cartera.I have about fifty euros in the wallet.
In the examples above, the number is invariable and it never changes its form.
Now, look at these examples:
Elías tenía unas cuatrocientas libras en el banco.Elías had about four hundred pounds in the bank.
El viaje ha sido unos doscientos kilómetros.The trip was approximately two hundred kilometers.
Note that in the examples above, the whole hundreds from 200 onwards (200, 300, 400, 500, 600, etc.) agree with the noun they precede. Be careful because with other hundreds like 203, 345, etc., the tens and units do not change, just the hundreds.
Había unas quinientas treinta y cuatro especies en peligro de extinción.There were about five hundred and thirty-four species in danger of extinction.
If we are talking about "certain quantities" of something, be careful to agree the article unos/unas with the noun immediately after. For example:
Usaron unos doscientos litros de agua para llenar la piscina.They used about 200 litres of water to fill up the pool.
The agreement is with litros, and not agua.
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