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Which one is correct and why? Please explain it to me.
◆ Guys you should take this test to find out which level you are in.
◆ Guys you should take this test to find out which level you are at.
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These are informal and casual statements. Both are used.

There should be a comma after "Guys".

+1

In both sentences, the word 'guys' is a vocative, ie a form of address, and should be followed by a comma.


Without further context, natural English is 'at'.

in is used more in an academic context where a level refers to a class.

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