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↩️ How Addition Undoes Subtraction

Addition and subtraction undo each other. Learn to use that connection to check your answers and crack missing-number problems.

3
lessons
~15 min
to learn
🔢 Math
subject
Ages 6–12
level
Start the course →

What you’ll learn

  1. Opposites That UndoUnderstand that addition and subtraction are opposites: subtracting a number undoes adding it.Adding and subtracting are opposite moves, like walking forward and back. Start at 9, add 5 to reach 14, subtract 5 and you're home at 9. That's why 8 + 6 = 14 and 14 − 6 = 8 belong to the same fact family — one walks the other backwards.
  2. Check Your WorkCheck an addition like 15 + 7 = 22 by working out 22 − 7 = 15, and check subtraction by adding back.The undo trick turns you into your own answer-checker. After 15 + 7 = 22, subtract the 7 from your answer: 22 − 7 = 15 lands back at the start, so the sum is right. It even catches mistakes — 18 − 6 = 12 proves 13 + 6 can't be 18. Subtractions get checked the same way, by adding back.
  3. Crack the Mystery NumberSolve missing-number problems like □ + 9 = 14 by using the inverse: □ = 14 − 9 = 5.When a mystery number has something added to it, undo the adding to set it free. For □ + 9 = 14, subtract: 14 − 9 = 5. Then check by putting the number back in: 5 + 9 = 14. Undoing plus checking makes you unbeatable at mystery numbers.

Questions this course answers

You start at 9, add 5, then subtract 5. Where do you end up?

Subtracting 5 undoes adding 5, so you land back on 9.

To check 15 + 7 = 22, what should you work out?

Subtract what you added from your answer: 22 − 7 = 15, back at the start, so it's correct.

Which subtraction checks that 8 + 6 = 14 is correct?

14 − 6 = 8 brings you back to the starting number, proving the addition.

Grounded in trusted sources

  • Khan Academy Kids
  • PBS Kids — early math
  • NAEYC — early math learning

Every Wunder lesson is built from real, reputable sources — never invented.

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