Be a Subtraction Detective: Use Addition to Check Your Answers!

Hello Maths Investigators! You’re getting really good at subtraction. But wouldn’t it be great to have a secret way to check if your answer is definitely correct? Well, there is! Because addition and subtraction are inverse operations (opposites that undo each other), you can use addition to check your subtraction calculations. It’s like having a maths superpower to confirm your work!

How Does the Addition Check Work for Subtraction?

It’s simple! When you subtract, you start with a bigger number, take some away, and get a smaller answer (the difference). To check your work using addition:

1. Take your answer (the difference).
2. Add back the number you subtracted.
3. If your subtraction was correct, you should get back to the number you started with!

Let’s See it in Action! Example: 50 − 20 = 30

• You did the subtraction: 50 − 20 = 30.
• Here, 30 is your answer (the difference).
• The number you subtracted was 20.
• The number you started with was 50.
• To check: Take your answer (30) and add the number you subtracted (20).
  • 30 + 20 = ?
  • 30 + 20 = 50.
• Compare: Is this the number you started your subtraction with? Yes, 50 is our starting number!
• Conclusion: This means your original subtraction (50 − 20 = 30) was correct! High five!

Another Example: Checking 75 − 15 = 60

• Subtraction: 75 − 15 = 60
• Your answer is 60.
• You subtracted 15.
• Check with addition: 60 + 15 = 75.
• Does this match your starting number? Yes! So, 75 − 15 = 60 is correct.

If your addition check doesn’t give you the starting number from your subtraction, it’s a clue that you might need to look at your original subtraction again!

Practice Being a Subtraction Checker! (18 Problems)

Ready to use your addition checking powers? For each problem below, you’ll see a subtraction sum. First, solve it. Then, use addition to check if your answer is correct! Or, we might give you a subtraction and its answer, and you have to do the addition check to verify it.

(Your web app with the 18 questions will go here. Questions should involve performing a subtraction, then doing the addition check, or verifying a given subtraction and its check.)

Why is Checking Subtraction with Addition So Smart?

• Boosts Accuracy: It helps you catch any little mistakes you might have made in your subtraction.
• Builds Confidence: Knowing you can check your work makes you feel more sure about your answers.
• Reinforces Understanding: It shows you really understand how addition and subtraction are connected.
• Makes You Independent: You don’t always need someone else to tell you if you’re right – you can check for yourself!

Tips for Grown-Ups: Encouraging the “Addition Check”

Teaching children to check their subtraction calculations using addition is a practical application of understanding inverse operations. It promotes self-correction and a deeper understanding of number relationships.

• Make it a Routine: After a child completes a subtraction problem, ask, “Great! Now, how can you use addition to check if that’s the right answer?”
• Use “Fact Family” Language: Remind them that if a − b = c, then c + b = a. “You found that 50 minus 20 is 30. So, what should 30 plus 20 be?”
• Model the Process: Work through examples together, verbalizing the check. “Okay, I think 45 − 12 is 33. Let me check… 33 + 12… yes, that’s 45! So I was right.”
• Focus on the “Why”: Explain that it works because you are simply “putting back” what you “took away.”
• Don’t Just Do It For Wrong Answers: Encourage checking even when the answer is likely correct. It reinforces the process and the relationship between the operations.