I took an English midterm. And the following is the part of it. I want to know a formal English not a colloqial one. It seems that everythig's are OK in colloquial usage. And, to the person who responds, thank you in advance.

According to the above story, Ida Lewis had saved at least 3 persons in trouble in the ocean.

I learned that '1 person', '2 people', '3people' and so on. And 3 persons are not usually used if it is in a specific case like a statistic or.... I am so confused. In a language school they emphasized that 'one person' or '2 people' What do you think?

In formal English, either three persons or three people would be OK. I don't think that's a very good question to put on a test.

Best wishes, Clive
You don't have to use 'people', though it is far more natural to do so, except in some legal or technical contexts.
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Thank you always!
I do understand your question.
Here is the answer.
In English one person is singular, but when you talk about more than 1 you have to use People which is the Plural of person.
1 Person
2 People
3 People
150 People
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 fivejedjon's reply was promoted to an answer.