**Prime numbers up to 100** *KS2 Revision*

## What you need to know

**Things to remember:**

- You need to know the prime numbers between 1 and 20
- 1 isn’t a prime number
- 2 is the only even prime number

*A prime number is a positive whole number bigger than 1 that only has two factors, itself and 1.*

For example, 13 is a prime number because its only factors are 1 and 13.

You need to learn the prime numbers between 1 and 20:

2 3 5 7

11 13 17 19

2 is a special prime, because it is the only even prime. All other even numbers can be divided by 2, so can’t be prime.

You need to be able to tell if a number smaller than 100 is prime.

**Is 59 a prime number?**

If you can’t divide a number less than 100 by 2, 3,5, or 7 and get a whole number, then it is prime.

59\div2=29.5

59 \div 3 = 19.6666...

59 \div 5 = 11.8

59 \div 7 = 8.42…

59 is a prime number!

## Example Questions

**Question 1:** Is 19 a prime number?

19\div2=9.5

19 \div 3 = 6.333...

19 \div 5 = 3.8

19 \div 7 = 2.714…

19 is a prime number.

**Question 2: **Is 42 a prime number?

42\div2=21

42\div 3 = 14

42 \div 5 = 8.4

42\div 7 = 6

42 is not a prime number.