**Prime numbers **are some of the most useful numbers in maths and they can help you out a lot!

Therefore, it is a really good idea to understand how to identify them and to commit, at least the most common ones, to memory.

**Definition:** A prime number is a number that only has two positive factors.

**What this means in reality is that you can only divide a prime number by two things: 1 and itself. **

So 17 would be a prime number because you can only divide it by 1 and 17.

But 15 wouldn't be a prime number because you can divide it by 4 things: 1, 3, 5 and 15.

Numbers that aren't prime numbers are called **composite numbers**.

**What are the most common prime numbers?**

You are expected to know the prime numbers up to 30, so you need to memorise this list:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29

In this activity, you will practise identifying prime and composite numbers so that you can use these facts to help you solve complex problems and mental computations quickly.