Which number is prime among 21, 27, 17, 33?

Which number is prime among 21, 27, 17, 33?

• A. 21
• B. 27
• C. 17
• D. 33

Answer: (C)

Prime numbers are numbers greater than 1 that have exactly two divisors: 1 and the number itself. Among these options, 21 is divisible by 3 and 7, so it has more than two divisors. 27 is divisible by 3 (and 9), so it isn’t prime. 17 can’t be divided evenly by any number other than 1 and itself; since the square root of 17 is about 4, you only need to test 2 and 3, and it isn’t divisible by either. That makes it prime. 33 is divisible by 3 and 11, so it isn’t prime.

Prime numbers are numbers greater than 1 that have exactly two divisors: 1 and the number itself. Among these options, 21 is divisible by 3 and 7, so it has more than two divisors. 27 is divisible by 3 (and 9), so it isn’t prime. 17 can’t be divided evenly by any number other than 1 and itself; since the square root of 17 is about 4, you only need to test 2 and 3, and it isn’t divisible by either. That makes it prime. 33 is divisible by 3 and 11, so it isn’t prime.