In this case, "you" is 15 people.
Are both of these correct?
I initially thought the second was correct, but I was told the first one is… Now I've looked at both of them too long to be able to tell 😅.
In British English you can never switch between pronouns in the same sentence, so whether or not the example here with they is ambiguous, it wouldn’t pass muster.
In American English it appears common to switch between one and you — the most usual alternatives — and this view is supported by this online article and this one.
I do not know whether American English allows switching between you and they, although in the example quoted, as already mentioned, it is ambiguous and therefore to be avoided.
I think it depends on what the speaker means. "Most of you would do the right think if you knew," indicates that the speaker is referring to the audience for the entire sentence; meaning that the audience would do 'something' if the audience knew 'something'.
To me, "Most of you would do the right thing if they knew," suggests a third party involved. For example, "Most of you(audience) would do the right thing if they(police/figure of authority/etc.) knew."
In my experiences, when addressing an audience, if the speaker uses a pronoun, (i.e. they, he, she, etc.), then it hasn't typically referred to anyone in the audience unless they say, "(Specific audience member(s)), they/he/she..." and so on. So, if the second "you/they" is referring to the audience specifically, and not a third party, then the correct sentence would be, "Most of you would do the right thing if you knew."
A perusal of the numerous examples afforded by the ngram "most of you would" shows that when the pronouns and possessives apply to the group identified by "you" in "most of you", it is never the case of the third person being chosen; it is always the pronoun "you" and the possessive determiner "your". (never "they", "them", "their").
This should perhaps not be taken as the last word on the question. That seems to be the most logical choice when the speaker thinks proper to impute the state of knowing to the whole group out of which a subgroup corresponding to "most of you" is defined. However, an anaphoric refernce to only the selected group, or in other words to "most of you" would appear to be properly made through the use of the third person pronouns and determiners.
This is the correct answer.
Most of you would do the right thing if you knew
You can't switch between you and they. The former refers to the people being addressed. The latter to other people not being addressed. The two terms have logically distinct referents.
That's the end.
