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

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Multiple Choice

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

Explanation:
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.

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