what do you call a person who spent 20 years in the 24th row in the theatre?

This is not a math question.

The term you are looking for is "a seasoned theatergoer." To arrive at this answer, I can infer that the question is referring to someone who regularly attends theater performances and has consistently sat in the same row for 20 years. By understanding the context and providing a generalized term, "a seasoned theatergoer" captures this idea.