Add and Subtract with Negative Numbers

Look at this number line: 

 

 

Generally, if we are adding we count to the right, and subtracting we count to the left. 

 

However, if we are dealing with negative numbers, there are some rules to remember. 

 

The table below summarises the rules:

 

Calculation involves	In words	This means I need to
+	Adding a positive	Count to the right
-	Subtracting a positive	Count to the left
+ -	Adding a negative	Count to the left
- -	Subtracting a negative	Count to the right

 

Example:

3 + -5 

We are adding a negative, so count to the left. Start on 3 and count 5 left. You should end up at -2.

so 3 + -5 = -2

 

Example:

6 - -7

We are subtracting a negative, so count to the right. Start on 6 and count 7 right. You should end up at 13

6 - -7 = 13

 

 

It can help if you know these rules for operation signs that appear next to each other:

 

+ and + = +

- and - = +

+ and - = -

- and + = -

 

Let's practise this now in the questions that follow.