to find given number is prime or not?

Answer 1

You only need to check for divisibility by prime numbers up to the square root of the number.

For example the square root of 101 is 10.05
To check whether 101 is prime or not you need to check primes less than 10.05
That's 2, 3, 5, and 7. None of these four numbers divides 101, so 101 is prime.

Answer 2

List all the factors in pairs. If there is only one pair of factors- 1 and itself- then it's a prime number.
Example-
7 factors (1,7) = prime
24 factors (1,24), (2,12), (3,8), (4,6) + not prime (composite)

Answer 3

keep dividing it by every number up to half it's value.