Question

How can we know that an number is prime or not

Answer 1

A prime number is a whole number greater than 1 whose only factors are 1 and itself.

A factor is a whole numbers that can bedivided evenly into another number.
The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29.Numbers that have more than two factors are called composite numbers.
The number 1 is neither prime nor composite.

Simply
A number is greater than 1 is called a prime number, if it has only two factors, namely 1 and the number itself
so we can find these types of numbers are prime numbers