Check subtraction with addition

To check a subtraction, add the answer to the number you took away. If you get the starting number, the answer is right.

How checking with addition works

Subtraction and addition are inverse operations: each one undoes the other. If you take 18 away from 52 and get 34, then putting the 18 back must give 52 again.

  1. Do the subtraction: 52 − 18 = 34.
  2. Add the answer to the number taken away: 34 + 18.
  3. If you get the top number (52), the subtraction is right. If not, look again.

This check takes a few seconds and catches most mistakes, especially in regrouping.

See it: pictures to draw with your child

Copy these onto paper or a whiteboard. A quick sketch makes the idea concrete before any written method.

5218taken away34left
52 is made of 18 and 34. So 52 − 18 = 34 and 34 + 18 = 52.
TO451312284−
Subtract first: 52 − 18 = 34.
304050+10+83452
Check: from 34, add 18 (a jump of 10 and 8) to land back on 52.

Why it works

A subtraction splits a whole into two parts: the part taken away and the part left. 52 − 18 = 34 says that 52 is made of 18 and 34. Adding the two parts must give the whole.

A bar model shows this clearly. The whole bar is 52. One part is 18, the other part is 34. Read it as a subtraction or an addition; it is the same picture. The fact families page uses the same idea for small numbers.

Subtract, then check

Step through the subtraction. The last step shows the addition check.

Two ways to check

CheckFor 92 − 37 = 55When to use it
Add back the number taken away55 + 37 = 92, correctEvery time: the standard check
Subtract the answer92 − 55 = 37, correctWhen adding feels harder
Estimate90 − 40 = 50, close to 55A quick sense check before or after

The first check is the strongest because it uses a different operation. If a child makes the same slip twice, two subtractions can agree and both be wrong; addition catches it.

What a failed check tells you

If the check gives the wrong number, the size of the error is a clue:

Teach your child to check before showing you an answer. It turns “is this right?” into something they can answer themselves.

Make checking a habit

Children often see checking as extra work. Make it part of the method: the written answer is not finished until the check is done. A quick way is to write the check under the answer in a different colour.

Checking also builds confidence. A child who knows how to check does not need an adult to say whether an answer is right, and that independence makes homework calmer for everyone. On tests, a 10-second check often turns a wrong answer into a right one.

Quiz: check the subtraction

Add the answer to the number taken away.

Where this fits: subtraction step by step

  1. What is subtraction
  2. Subtraction within 10
  3. Counting back
  4. Subtraction on a number line
  5. Counting up
  6. Fact families
  7. Make 10 subtraction strategy
  8. Subtraction within 20
  9. Subtracting tens and hundreds
  10. 2-digit subtraction
  11. Subtraction with regrouping
  12. 3-digit subtraction
  13. Subtracting across zeros
  14. Mental subtraction strategies
  15. Checking subtraction with addition (this page)
  16. Missing number subtraction
  17. Estimating differences
  18. Multi-digit subtraction
  19. Subtraction word problems

What to say to your child

A short script for teaching it the first time. Pause after each line and let your child do the next step.

  1. “You worked out 52 − 18 = 34. How could we be sure?” (Not sure.)
  2. “If we took 18 away, what happens if we put it back?” (We get 52 again.)
  3. “Let’s add: 34 + 18.” (52!)
  4. “So the subtraction is right. That’s checking with addition.”
  5. “From now on, check before you show me.”

More worked examples

Example 1: a correct answer: 81 − 46 = 35

Add back: 35 + 46 = 81. That is the top number, so the answer is right.

Example 2: a wrong answer: 63 − 27 = 46

Add back: 46 + 27 = 73. That is 10 too many, so a regroup was missed. Redo: 63 − 27 = 36, and 36 + 27 = 63.

Example 3: 3 digits: 504 − 268 = 236

Add back: 236 + 268 = 504. Correct.

Practice questions (answers below)

Ask these out loud or write them down. Check each answer, and ask “how did you get that?” for one or two.

  1. Check 15 − 7 = 8.
  2. Check 40 − 16 = 24.
  3. Check 75 − 38 = 47.
  4. Check 92 − 45 = 47.
  5. Check 300 − 125 = 175.
  6. Check 642 − 358 = 384.
  7. What addition checks 56 − 19 = 37?
  8. Leo says 70 − 26 = 54. Is he right?
  9. Stretch: check 1,000 − 457 = 543.
Show the answers
  1. 8 + 7 = 15, correct
  2. 24 + 16 = 40, correct
  3. 47 + 38 = 85, wrong: it is 37
  4. 47 + 45 = 92, correct
  5. 175 + 125 = 300, correct
  6. 384 + 358 = 742, wrong: it is 284
  7. 37 + 19 = 56
  8. No: 54 + 26 = 80; the answer is 44
  9. 543 + 457 = 1,000, correct

Common mistakes

Adding the wrong two numbersChecks 52 − 18 = 34 with 52 + 34.Fix: add the answer and the number taken away: 34 + 18.
Accepting a close checkGets 82 instead of 92 and says “nearly”.Fix: the check must give exactly the top number; off by 10 means a regroup slip.
Only checking hard onesSkips the check on easy problems.Fix: checks take seconds; make it a habit for every answer.

How to teach it at home

Check my work

Make a deliberate mistake in a subtraction and ask your child to catch it.

Two-way flash cards

For each subtraction card, say the matching addition.

Shop receipts

Check the change on a receipt by adding it to the price.

By grade

More for each grade: Grade 1 math and the CBSE class 1 maths guide; Grade 2 math and the CBSE class 2 maths guide; Grade 3 math and the CBSE class 3 maths guide.

Worksheets and practice

Addition and subtraction worksheet, page 1

Addition and subtraction worksheet: printable PDF with an answer key. · Make a fresh test

Subtraction practice · subtraction speed test · subtraction chart · subtraction for kids

Last reviewed

Questions parents ask

How do you check subtraction with addition?

Add the answer to the number that was taken away. If you get the starting number, the subtraction is right: 52 − 18 = 34 because 34 + 18 = 52.

Why does addition check subtraction?

They are inverse operations. Subtraction splits a whole into two parts; adding the parts back gives the whole.

What does it mean if my check is off by 10?

Usually a regroup in the tens was missed or done twice. Look at the tens column again.

What grade learns to check subtraction with addition?

It starts in grade 1 with fact families (1.OA.B.4) and is used in grades 2 and 3 for bigger numbers.

Can you check addition with subtraction too?

Yes. If 34 + 18 = 52, then 52 − 18 should be 34.