Q. Why isn't 27 a prime

The number 27 is not prime because it has divisors other than 1 and itself—specifically, 27 = 3 × 3 × 3, or 3³. Since 27 can be evenly divided by 3 (27 ÷ 3 = 9) and by 9 (27 ÷ 9 = 3), it fails the definition of a prime number, which requires having exactly two distinct positive divisors. The number 27 is composite and is particularly notable as a perfect cube, being the cube of 3.

What makes 27 clearly non-prime is its multiple factors: the divisors of 27 are 1, 3, 9, and 27, giving it four divisors total rather than the two required for primality. The factorization 3 × 3 × 3 shows that 27 is built entirely from one prime number (3) repeated three times, making it a prime power but not itself prime. This contrasts with actual prime numbers in the same range like 23, 29, or 31, which cannot be broken down into smaller whole-number factors. Understanding why 27 isn't prime helps illustrate the difference between prime numbers (building blocks of all integers) and composite numbers (products of primes). Recognizing that any number expressible as a product of smaller integers greater than 1 must be composite is fundamental to number theory and primality testing.