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
1 · Read
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.
More to read
Add and subtract within 20: FAQ
Adding and taking away within 20 is a skill you will use every day. When the numbers are small, you can count on, count back, or make a ten to jump fast. Ten frames and number lines are handy tools that show each step. Pick the tool that feels easiest for the numbers in front of you. Subtraction asks what is left, or what is missing. If 8 + 6 = 14, then 14 take away 8 leaves 6. Fact families stick together, so learning one fact teaches you its partners for free.
2.5: Solving Equations Using the Subtraction and Addition Properties of Equality (Part 1)
Equations with a missing number are puzzles with one hidden piece. Addition and subtraction undo each other, so use the opposite move to reveal it. If something plus nine makes fourteen, take nine from fourteen to find the five. Always test your answer by placing it back into the puzzle. When both sides match, the puzzle is solved.
2.6: Solving Equations Using the Subtraction and Addition Properties of Equality (Part 2)
Equations with a missing number are puzzles with one hidden piece. Addition and subtraction undo each other, so use the opposite move to reveal it. If something plus nine makes fourteen, take nine from fourteen to find the five. Always test your answer by placing it back into the puzzle. When both sides match, the puzzle is solved.
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2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 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.