**Composite Number** *KS3 Revision*

## What you need to know

**Things to remember:**

- If a number has more than two factors it is called a “composite number”.
- If a number only has two factors, then it is called a “prime number”
- 1 isn’t prime, and it isn’t composite.

Before starting, we need to just quickly remind ourselves of what factors are:

A factor is a number that divides another number and gives a whole number. For example, because 32\div8=4, then 8 is a factor of 32. This means that 4 is a factor of 32 as well. We call these two numbers, 4 and 8, a factor pair. We can also think of the factor pairs as multiplying:

4\times8=32

Previously we have look at prime numbers and found that they only had two factors, 1 and itself.

Prime Number Factors

2 1 and 2

3 1 and 3

5 1 and 5

7 1 and 7

11 1 and 11

13 1 and 13

17 1 and 17

19 1 and 19

So, we have a special name for numbers with 2 factors, but how about numbers with more than two factors? We call these “composite numbers”.

Composite Number Factors

4 1, 2, and 4

9 1, 3, and 9

12 1, 2, 3, and 4

18 1, 2, 3, 6, 9, and 18

26 1, 2, 13, and 26

So, these are clearly composite numbers because they have more than two factors.

Let’s look at a special case, 1:

Number Factors

1 1

So, 1 only has one factor, itself. So, 1 isn’t prime and it isn’t composite.

## Example Questions

**Question 1:** Is 24 prime or composite?

Factors of 24

1 2 3 4 6 8 12 24

24 is a composite number.

**Question 2:** Is 29 prime or composite?

Factors of 29

1 29