This is my sentence:
90% of people age 18 or under have a smartphone.
My questions:
I know with 90% you gotta use have. But do I use a smartphone or smartphones. I heard a like of people say it like this. But we know 90% can't be just one person, so say there are 100 people in my class age 18 or under. And I say 90% of them have a smartphone. That can't be right because 90% of 100 is 90. And it is impossible for 90 people to own one smartphone.
90% of people aged 18 or under have a smartphone is correct because you are using the indefinite article when referring to smartphones, which means you are not referring to any specific smartphone.
Under the first use case here, it states:
A and AN are called indefinite articles. "Indefinite" means "not specific". Use A(AN) when you are talking about a thing in general, NOT a specific thing.
Since you are talking about owning a smartphone in general, this would make sense. If the statement was "90% of people aged 18 or under have the smartphone", then your presumption would be correct, as using the refers to a single, previously defined smartphone.
'90% of people age 18 or under have a smartphone', is correct usage. This does not mean "90% of people age 18 or under are sharing a single smartphone'. It means something more like, '90% of people age 18 or under have a smartphone of one type or another'. However, the additional text is not required because, for the reasons given in the answer from The Real Lester, most English speakers will understand what is meant from the context of the statement.
Of these 90% of people 18 or under, it is likely that a lower percentage will have more than one smartphone. I don't know that actual percentage, but lets assume it is 30%. In this case you could say,:
90% of people age 18 or under have at least one smartphone.
and
30% of people age 18 or under have more than one smartphone.
However, you would need to say:
30% of people age 18 or under have at least two smartphones.
