# Classify prime or composite (below 100)

Lesson

## Prime or composite?

Let's look at some of the rules to help us work out whether a number is prime or composite.

## Prime numbers

A prime number must be:

• a whole number
• a number greater than $1$1
• only divisible evenly by itself, and $1$1

To be divisible evenly, it means that when we divide by a factor, we don't have any remainder.  In Video 1, we'll look at some numbers to work out if they are prime numbers.

## Composite numbers

Even though most whole numbers that are not prime numbers are composite numbers, there are some exceptions. First though, let's see what defines a composite number. A composite number must be:

• a whole number
• a number greater than $1$1
• divisible (with no remainders) by other whole numbers, as well as itself and $1$1

In Video 2, we look at how to check if a number is a composite number.

Remember!

$2$2 is the only even number that is NOT a composite number.

## Neither prime nor composite

Even though $1$1 is divisible only by itself and $1$1, it is not a prime number because it is equal to $1$1, not greater than $1$1. In the same way, the number $0$0 also can't be defined as prime or composite. Remember that one of the rules for both prime and composite numbers is that they must be greater than $1$1.

## Checking numbers

When we need to determine if a number is a prime or composite number, we can use some things we already know to help us.

Think about the number $70$70. We know that $70\times1=70$70×1=70, but we also know that $7\times10=70$7×10=70, so $70$70 has factors other than $1$1 and itself, which means it must be a composite number. Watch Video 3 to see what other strategies you can use to decide.

To explore factors in more detail, you may wish to review this topic on divisibility tests.

Remember!
• $0$0 and $1$1 are neither prime nor composite numbers, as they don't satisfy either definition.
• Every whole number greater than $1$1 can only be a prime number or a composite number.
• The number $2$2 is the only even number greater than $1$1 to be a prime number. It's also the only even number greater than $1$1 that is not a composite number.

#### Practice questions

##### Question 1

Is $2$2 a prime or composite number?

1. Prime

A

Composite

B

##### Question 2

Is $9$9 a prime or composite number?

1. Prime

A

Composite

B

##### Question 3

True or False?

$54$54 is a prime number.

1. True

A

False

B