Which one of the following is not a prime number?

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Which one of the following is not a prime number? Answer: 91

Question context

This question is part of 📏 Arithmetic Aptitude within the broader 🧠 Aptitude Mastery subject and appears in the DemoPaper-Pakistan preparation collection. Reviewing the answer together with nearby questions helps reinforce the underlying concept and identify how it may appear in competitive exams.