Pre-Algebra · Quick Reference

Prime Numbers Cheat Sheet

Pre-Algebra · Lesson 8/10

In one line: a prime number can only be divided evenly by 1 and itself — these special numbers are the basic building blocks every other whole number is made from.

Key Ideas

1Prime Number. A whole number greater than 1 with exactly two factors: 1 and itself. Examples: 2, 3, 5, 7, 11, 13.

2Composite Number. A whole number greater than 1 with more than two factors, meaning it breaks down into smaller factors.

3The Number 1 is Special. 1 is neither prime nor composite, since it has only one factor (itself).

4The Only Even Prime. 2 is the only even prime number — every other even number is divisible by 2, giving it a third factor.

5Quick Primality Check. To check if a number is prime, test whether any prime number up to its square root divides it evenly; if none do, it's prime.

Worked Examples

Is 13 a prime number?

Yes, 13 is prime

Is 27 prime or composite?

27 is composite

List all prime numbers between 1 and 20.

2, 3, 5, 7, 11, 13, 17, 19

Practice Yourself

Is 11 prime?

Yes

Is 21 prime or composite?

Composite (3x7)

Is 1 prime, composite, or neither?

Neither

What is the only even prime number?