Which number is prime: 25, 27, 29, 21

Which number is prime: 25, 27, 29, 21

• A. 25
• B. 27
• C. 29
• D. 21

Answer: (C)

Prime numbers are numbers greater than 1 that have exactly two divisors: 1 and the number itself. To test 29, check for factors up to the square root of 29 (about 5.38). It isn’t divisible by 2 (it's odd), by 3 (the digits add to 11, not a multiple of 3), or by 5 (it doesn’t end in 0 or 5). Since no smaller factor works, the only divisors are 1 and 29, so 29 is prime. The other numbers have obvious factors: 25 is 5×5, 27 is 3×9 (or 3×3×3), and 21 is 3×7, so they are not prime.

Prime numbers are numbers greater than 1 that have exactly two divisors: 1 and the number itself. To test 29, check for factors up to the square root of 29 (about 5.38). It isn’t divisible by 2 (it's odd), by 3 (the digits add to 11, not a multiple of 3), or by 5 (it doesn’t end in 0 or 5). Since no smaller factor works, the only divisors are 1 and 29, so 29 is prime. The other numbers have obvious factors: 25 is 5×5, 27 is 3×9 (or 3×3×3), and 21 is 3×7, so they are not prime.