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Which of the following is not a prime number?
$A.$ 31
$B.$ 61
$C.$ 71
$D.$ 91

Last updated date: 24th Jul 2024
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Hint: - Here we go through the properties of the prime number for finding the prime number we have to first find out the factors of the number. Then by property of prime we find out the answer. A prime number has only 1 and itself as factors.

Complete step-by-step answer:

As we know a prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A simple but slow method of checking the primality of a given number n, by calculating the factors of n. if the factor is only in the form of $1 \times n$ then it is prime otherwise it is not a prime.
Now let us calculate the factors,
  31 = 1 \times 31 \\
  61 = 1 \times 61 \\
  71 = 1 \times 71 \\
  91 = 1 \times 7 \times 13 \\
Here we can clearly see that 91 is not a prime because its factor is more than one and itself.\
Hence option D is the correct answer.

Note:- Whenever we face such a question to find out if the number is prime or not we have to go through the definition of prime number if the number satisfies the definition then the number is prime otherwise it is not a prime.