How do you know a number is prime?

Answer 1

Prime numbers are not divisible by any integer other than themselves and one. So if you can't produce an integer (whole number) by dividing the number by any integer, it's prime. For example, 13 is a Prime number. You can't produce 13 by multiplying any integers together except 13 and 1.

Answer 2

A prime number is a positive integer with two factors: one and the number itself. If you test the numbers up to the square root and your number is not divisible by any of them, it's prime.