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.
For example:
In the examples above, the number is invariable and it never changes its form.
Now, look at these examples:
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.
For example:
Extra note:
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.
See also:
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