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The instruction in your post is IRRELEVANT and INCORRECT.
It only CONFUSE the reader and is a REAL OBSTACLE to the solution.
It makes VERY BAD IMPRESSION about the person who composed this problem,
because it shows his (or her) EXTREMELY LOW understanding/knowledge of the subject and the problem.
Solution
The idea is to subtract from the total number of all possible permutations of 4 objects
the number of permutations where persons A and B are together.
The total number of permutations of 4 objects is 4! = 4*3*2*1 = 24.
To determine the number of permutations of 4 persons where A and B are together, we consider A and B as one object,
but we shoud account that A and B can sit in two different configurations (AB) and (BA).
Therefore, the answer to the problem's question is
4! - 2*3! = 24 - 2*6 = 24 - 12 = 12.
Solved.