What is a Prime Number
Prime number is an integer number greater than 1 that has only two factors: 1 and itself.
For example, 5 is prime number because it has no other factors except 1 and 5.
By convention 1 is not prime, so prime numbers start from 2.
Interesting fact. 2 is the only even prime number. Other even numbers are not prime, because they have factor 2.
So, prime numbers are following: 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,...
Example 1. Determine whether 25 is prime.
Not prime because 25 is divisible by 5, so 5 is factor of 25.
Example 2. Determine whether 61 is prime.
It is prime number because it has only two factors: 1 and 61.
Now, practice a bit.
Exercise 1. Determine whether 71 is prime.
Exercise 2. Determine whether 75 is prime.
Answer: not prime.