How do you find a number is prime or composite?

Answer 1

One way is to divide by all smaller numbers, up to the square root of that number. In practice, you can omit even numbers (except 2), and any number which you know is prime. Thus, for any number up to 120, it is enough to try to divide by 2, 3, 5, 7. If any of these divisions yields an integer, then the number you divided by is a factor - and your original number is composite.

Answer 2

Factor it. Prime numbers have two factors, composite numbers have more than two.

Answer 3

There is no simple way to tell. To find out if a number N is prime, you have to test for divisibility by every Prime number up to int(sqrt(N)) = the integer part of the square root of N.