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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.

Q: Which number is prime out of 91 51 31 21?

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31 is a prime number.

31

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

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