Question

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

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.

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.
$31 = 1 \times 31 \\ 61 = 1 \times 61 \\ 71 = 1 \times 71 \\ 91 = 1 \times 7 \times 13 \\$