What are related subtraction facts
Related subtraction facts are number sentences that use the same three numbers. They show all the different ways those numbers can be combined in a subtraction problem.
- If you know that 14 − 5 = 9, then a related fact is 14 − 9 = 5.
- The three numbers are 14, 5, and 9. They stay the same, but their order in the problem changes.
Think of it like a fact family! The three numbers belong together, and you can use them to build more than one subtraction sentence.
The whole and the parts
In a subtraction fact, you start with the whole (the biggest number). You take away one part to find the other part.
- In 12 − 4 = 8:
12 is the whole.
4 is one part.
8 is the other part.
The first number in a subtraction sentence is always the whole. When you find related facts, you are just swapping which part you take away from the whole.
How to find a related fact
If you know one subtraction fact, you can write its related fact by taking the answer from the first fact and making it the number you subtract in the new fact. The whole stays the same.
- Start with: 15 − 7 = 8
Related fact: 15 − 8 = 7 - Start with: 19 − 11 = 8
Related fact: 19 − 8 = 11
Remember: Whole − Part = Other Part. For the related fact, it becomes Whole − Other Part = Part.
Using related facts to check your work
Related facts are helpful tools. You can use one fact to check if the answer to another fact is correct. If both facts are true with the same three numbers, your work is accurate.
- Is 17 − 9 = 8 correct?
Check with the related fact: 17 − 8 = 9.
Since that is also true, the first answer is correct. - Is 13 − 5 = 9 correct?
Check with the related fact: 13 − 9 = 4, not 5.
This shows the first answer is wrong. The correct fact is 13 − 5 = 8.
Related facts are like math partners—they help each other out! Using them to check answers is a smart strategy.
Related facts with larger numbers
The same rule works for all numbers. When the whole is a larger number like 18, 16, or 10, you can still find its related subtraction facts using the same three numbers.
- Whole = 18: 18 − 10 = 8 and 18 − 8 = 10
- Whole = 7: 7 − 3 = 4 and 7 − 4 = 3
- Whole = 16: 16 − 9 = 7 and 16 − 7 = 9
No matter the size of the numbers, if you have the whole and one part, you can always find the related fact. This skill helps make you faster and more confident with subtraction.
Common Core alignment: CCSS.MATH.CONTENT.1.OA.B.4 – Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
Notes for teachers
This free lesson is aligned with CCSS.MATH.CONTENT.1.OA.B.4. Use it for whole-class instruction, independent practice, or homework.
The lesson builds the foundational understanding that subtraction facts come in related pairs, reinforcing the part-whole relationship crucial for fluency and future work with fact families and missing-addend problems. All content is 100% free, student-safe, and designed for classroom and home use.