Inverse: addition undoes subtraction
Recognise and use the inverse relationship between addition and subtraction to check calculations and solve missing-number problems
What a learner can do afterwards
- Check 15 + 7 = 22 by calculating 22 − 7 = 15
- Use the inverse to solve: □ + 9 = 14, so □ = 14 − 9 = 5
- Explain that addition and subtraction 'undo' each other
The lesson
Addition and subtraction are opposites. If you add a number and then subtract that same number, you end up back where you started. This is called the inverse, and it lets you check your answers.
Mia works out 15 + 7 = 22. To check her answer, she does the inverse: 22 − 7. She gets 15 again, the number she started with, so she knows her addition was right.
You can use the same trick to find a missing number. If □ + 9 = 14, undo the addition: 14 − 9 = 5. So the missing number is 5.
Addition and subtraction undo each other, so you can use one to check or find the other.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.