Which number is not composite 2,9,15,21
A composite number is divisible by other numbers, including 1 and itself. Opposite of composite is prime, so we need to look for the PRIME number.2 is a prime number because it is only divisible by 1 and itself(2). Answer: 2
2 is the only number that is prime. The others are composite because they have factors other than 1 and itself.