Show that 21 is not a prime number.

A prime number has only two factors i.e 1 and the number itself

Factors of 21 are 1,3,7,21

So 21 have more than 2 factors and that does not satisfy the condition of a prime number.

Hence 21 is not prime

For a number to be classified as a primary number it should have two factors. 21 has more than two factors i.e. 1, 3, 7, 21! So it is not prime!