# Prime Number Formula

When a number is divisible by only one and itself, then it is a prime number. The prime numbers cannot be factorised as they are not divisible. The divisible numbers are called composite numbers. 1 is neither prime nor composite.

How do we get to know if a number is prime or not?

To calculate we need to find the factors of the number. Take for example 11, 11 is divisible only by 1 and itself. Since it has only two factors, it is a prime number.

### Solved Example.

Question 1: Find if 51 is a prime number or not ?

Solution.

The factors of 51 are 1 and 51.
So 51 is only divisible by 1 and 51.
So, 51 is a prime number.

Question 2: Check if 64 is a prime number or not?

Solution. The factors of 1, 2, 4, 16, 8.
Hence it is a composite number and not a prime number.

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