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|
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
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.