Indefinite article in the "An [adjective] [number] [plural noun]" construction

I wasn't sure how best to phrase the title of this question.

I'm interested in constructions of the following form:

An estimated 50 people died in the bombing.

'An estimated' could be substituted with adverbs like 'approximately' or 'about', so it seems that prepending the indefinite article to certain words produces an adverb.

My question is why the indefinite article should function in this way.

For one thing, the use of 'a[n]' when the following noun is generally plural seems odd.

There are quite a few words that fit this pattern:

• An additional ...


• An approximate ...


• An estimated ...



• An extra ...


• A good ...


• A huge ...


• A mere ...


• A possible ...


• A record ...


• A scant ...


• A whopping ...

Answer

Dictionary.com's fifth definition of a is:

5. indefinitely or nonspecifically (used with adjectives expressing number): a great many years; a few stars.

This usage of a is standard and often seen in this type of context.