31 is a prime number.
31
31
31 is the only one that has only two factors: 1 and itself. It is the prime number.
51 is not a prime number: 3x17=51
21 is divisible by 3 and 7, so it is composite. 91 is divisible by 7 and 13, so it is composite. 51 is divisible by 3 and 17. 31 is divisible by only 1 and itself, so it is the prime number.
51 is composite number
91 = 13*751 = 17*321 = 3*731 is prime. â–
51 is composite.
31 is prime, but 51 is not as 51=3*17.
51 is not a prime number. It is divisible by 3.
51 = 3 x 17