Adding undoes subtracting. That gives you a way to check yourself.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
How to check
If 62 − 27 = 35, then 35 + 27 should give 62 back.
If it does not
You have found a mistake, and you found it yourself. That is worth doing.
Why adding back works
Subtraction removes an amount; adding it back returns you to where you started. If 63 - 28 = 35, then 35 + 28 must be 63. Any answer that fails this test is wrong, and the test is genuinely independent — you are doing a different operation, not just repeating the same steps.
Make it automatic
Get into the habit of checking every subtraction this way, not just the ones you feel unsure about. The errors you are most confident about are exactly the ones you would never think to check.
A second, faster check
Estimating is quicker but rougher. 63 - 28 is about 60 - 30 = 30, so an answer near 30 is plausible and an answer of 45 is not. Use estimation to catch big errors fast, and adding back to confirm the exact answer.
Step 2: Try It Yourself
Tap and try it out.
62
35
27
- 35 + 27 = 62
Step 3: Watch an Example
One step at a time.
Watch Leo Catch a Mistake
Leo works out 73 − 48 and gets 35.
- Step 1
He checks by adding: 35 + 48.
Step 4: Your Turn
Practice makes it stick.
Checking Up
Problem 1 of 2
Ana says 84 − 29 = 55. Check it: what is 55 + 29?
A Wrong Answer
Problem 2 of 2
Leo says 52 − 38 = 24. What is 24 + 38?
Work It, Then Check It
1 of 4
61 − 34 = ?
2 of 4
Check it: what is 27 + 34?
3 of 4
90 − 47 = ?
4 of 4
Check it: what is 43 + 47?
Step 5: Quick Check
Show what you know.
Question 1 of 2
75 − 38 = ?
Question 2 of 2
How do you check a subtraction?
What You Learned
- Adding undoes subtracting.
- Add your answer to what was taken away.
- If you get back to the start, you were right.