Concepts

A prime number is a whole number that only has two factors, which are itself and one. 
A composite number has more than two factors.

Rules

• The numbers 0 and 1 are neither prime nor composite.

• All even numbers are divisible by two, and so all even numbers greater than two are composite numbers.

• All numbers that end in five or zero are divisible by five. Therefore, all numbers that end with five and zero that are greater than five are composite numbers.

• The prime numbers between 2 and 100 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 and 97.

Example

Is this number prime or composite?

18

Solution

Find the factors of the number. If the number has more than 2 factors, then it is a composite number.
   
The factors of 18 are: 1, 2, 3, 6, 9, 18
Since there are more than two factors, 18 is a composite number.

Pre-requisite Skills

Identify Factors (5.10.1)