Same Math › Grades 1–3 › Think addition to subtract
What is think addition?
Think addition solves a subtraction problem by turning it around. For 13 − 8, ask "8 plus what makes 13?" The answer, 5, is the same.
- 13 is the whole. 8 is the part you know.
- The ? is the missing part. What goes with 8 to make 13?
- 8 + 5 = 13, so 13 − 8 = 5
Why schools teach it
Kids use addition facts they already know
First graders are expected to understand subtraction as an unknown-addend problem: 10 − 8 is finding the number that makes 10 when added to 8 (1.OA.B.4). Addition facts tend to stick better, so this turns hard subtraction into easy addition.
It's the same idea as fact families and counting up on the open number line. By the end of 2nd grade, kids should know subtraction within 20 from memory (2.OA.B.2), and this is how many get there.
At the kitchen table
What to say, and what to skip
Say this
What plus 8 makes 13?
You know 8 + 5. How does that help with 13 − 8?
Count up from 8 to 13. How many jumps?
Another way to find the missing part.Can you check by adding?
8 + 5 = 13, so the answer is right.
Skip this
- Counting back by ones for big numbers.13 − 8 by counting back 8 is slow. Counting up 5 is faster.
Subtraction and addition are different things.
Showing they're linked is the whole point.- Writing the answer as the bigger number.The missing part is 5, not 13.
See it in action
Solving 15 − 9 by thinking addition
Turn it around
9 + ? = 15Use a fact you know
9 + 6 = 15That's the answer
15 − 9 = 6
Kids who know 9 + 6 get 15 − 9 for free.
Free practice sheet
Make a think addition sheet
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Parents also ask
Is this the same as counting up?
Very close. Counting up finds the missing part by jumping from the smaller number to the bigger one.
Why not just count back?
Counting back works for small amounts. For 13 − 8, counting up 5 is much faster than counting back 8.
Does this work for bigger numbers?
Yes. 62 − 38 is 38 + ? = 62, which is easy on an open number line.
How is this connected to fact families?
Fact families show that 8 + 5 = 13 and 13 − 8 = 5 are the same three numbers.