Multiple choice general knowledge math & puzzles Which one of the following is not a prime number? 31 61 91 71 Reveal answer Fill a bubble to check yourself C Correct answer Explanation A prime number has only two factors: 1 and itself. 31, 61, and 71 are prime. However, 91 is composite because $7 \times 13 = 91$. Therefore, 91 is the correct choice as it is not a prime number.