Question

Which one of the following numbers is not a prime number?

• A. 241
• B. 337
• C. 391
• D. 571

Answer 1

The Correct Option is C

To determine which number is not a prime number, we need to check each option for divisibility other than by 1 and itself. Let's analyze the given options:

1. 241: The divisibility test shows that 241 is not divisible by any prime number up to √241 (which is approximately 15.5), such as 2, 3, 5, 7, 11, and 13. As it is not divisible, 241 is a prime number.
2. 337: Again, testing divisibility by all prime numbers up to √337 (approximately 18.4), like 2, 3, 5, 7, 11, 13, and 17, confirms it is not divisible. Thus, 337 is a prime number.
3. 391: Check for divisibility by primes up to √391 (approximately 19.8). Dividing by 17, we find that 391 ÷ 17 = 23, indicating that 391 is divisible by 17. Therefore, 391 is not a prime number.
4. 571: Check primes up to √571 (approximately 23.9). It is not divisible by 2, 3, 5, 7, 11, 13, 17, 19, and 23, demonstrating that 571 is also a prime number.

Therefore, among the options given, 391 is not a prime number.