Subtraction and addition are partners. To find $13 - 8$, think: 8 plus what makes 13? You know how to make a ten, so count up through 10.
Key idea
$13 - 8$ asks: what goes with 8 to make 13? Count up from 8 to 13.
Why it works
$13 - 8$ is the part left when 8 is taken from 13. That is the same part you add to 8 to get back to 13.
Worked example: Think addition
What is $13 - 8$?
- Think $8 + \square = 13$.
- 8 up to 10 is 2. 10 up to 13 is 3.
- $2 + 3 = 5$, so $13 - 8 = 5$.
Answer: 5
Worked example: Another one
What is $15 - 9$?
- Think $9 + \square = 15$.
- 9 up to 10 is 1. 10 up to 15 is 5.
- $1 + 5 = 6$, so $15 - 9 = 6$.
Answer: 6
Common mistake
Counting back 8 steps one at a time. That is slow, and easy to miscount. When the number you take away is big, count up instead.
For grown-ups: Ask subtraction facts as addition questions: “8 and what make 13?”