This stems from a discussion over on ELL which has moved beyond being useful to second-language learners. In short, consider the sentence:
It weighs about 5 pounds.
What part of speech is "about?"
Since the verb "weigh" is not having any effect on the "about five pounds," I think it must be intransitive. That leaves the "about five pounds" as a long adverbial phrase, modifying the verb, right?
If that is correct, then isn't "about" an adverb modifying the adverbial noun "pounds?"
Would the answer hold true if we were talking about the price tag on something?:
This shirt costs about 5 pounds.
Best Answer
Traditionally, words modifying just about anything other than a noun phrase were lumped into the default category: adverbs.
Here, a more analytical approach is to label about as a quantifier modifier, which is obviously its function (if one is in the 'numbers are quantifiers' camp. Those who define numbers as being different from quantifiers on the grounds of precision will find a difficulty with this.)
Collins certainly recognises numbers used before noun phrases as determiners:
seventy
determiner
...
6. a. amounting to seventy: the seventy varieties of fabric.