Fifteen is not a prime number. A prime number is a positive number that has only two possible divisors, 1 and itself. Fifteen can be divided by 3 and 5 as well as 1 and itself.

The prime factors of 50 are 2 and 5, with the rest of its factors not being prime numbers. A factor is any number that can be divided evenly into the base number of interest, but only some factors are also prime numbers.

A number that is not prime is called a composite number. Composite numbers can be divided up into smaller whole numbers called factors. A prime number has no factors other than one and itself.


Sixty-one is a prime number. It is the 18th prime number in the series. It is also a centered square number, a centered hexagonal number and a centered decagonal number and the sum of two square numbers.

31 is a prime number. This is due to the fact that it is only divisible by itself and the number one, which is the criteria for any prime number.

Eighty-nine is a prime number. To check for a prime number, divide it by every number from two to one less than the number (88 in this case). If no smaller number can be evenly divided into the number, which is the case ...

The number 51 is not a prime number; it is a composite number. A prime number is any positive number that can be divided evenly only by the number 1 and itself.