Addition Detective at Work: Using Subtraction to Check Your Sums!

Hello Brilliant Mathematicians! You know how addition and subtraction are like opposite buddies? We’ve learned how addition can check subtraction, and guess what? It works the other way too! You can use subtraction as a super-detective tool to check if your addition calculations are correct. It’s another way to be sure your answers are spot on!

How Does the Subtraction Check Work for Addition?

It’s all because they are inverse operations – they undo each other! When you add two numbers together, you get a total (the sum). To check your work using subtraction:

1. Start with your answer (the sum).
2. Subtract one of the original numbers you added.
3. If your addition was correct, you should get the other original number you added!

Let’s See it in Action! Example: 25 + 15 = 40

• You did the addition: 25 + 15 = 40.
• Here, 40 is your answer (the sum).
• The numbers you added were 25 and 15.
• To check (Method 1): Take your answer (40) and subtract one of the original numbers, say 15.
  • 40 − 15 = ?
  • 40 − 15 = 25.
• Compare: Is this the other number you started your addition with? Yes, 25 was the other number!
• Conclusion: This means your original addition (25 + 15 = 40) was correct!
• To check (Method 2 – you can also do it this way!): Take your answer (40) and subtract the other original number, 25.
  • 40 − 25 = ?
  • 40 − 25 = 15.
• Compare: Is this the other number you started your addition with? Yes, 15 was the other number!
• Conclusion: This again shows your original addition (25 + 15 = 40) was correct!

If your subtraction check doesn’t give you one of the original numbers you added, it’s a signal that you might want to re-check your original addition sum!

Practice Being an Addition Checker! (18 Problems)

Ready to put on your detective hat and use your subtraction checking powers? For each problem below, you might see an addition sum. First, solve it (if needed). Then, use subtraction to check if the answer is correct! Sometimes we might give you an addition and its answer, and you just do the subtraction check.

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

Why is Checking Addition with Subtraction a Top Skill?

• Finds Your Mistakes: It’s a great way to catch any slip-ups in your adding.
• Makes You More Confident: Knowing you can double-check your work helps you trust your maths skills.
• Shows You Understand Number Families: You see how three numbers can link together in addition and subtraction (fact families).
• Helps You Think Flexibly: You learn to look at problems from different angles!

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

Teaching children to check their addition calculations using subtraction reinforces their understanding of inverse operations and fact families. It’s a valuable self-monitoring skill.

• “How Else Could We Write That?”: After an addition sum, ask, “If we know these two small numbers make this big number, what subtraction sums can we also make with these three numbers?”
• Model Both Subtraction Checks: For a sum like a + b = c, show them that c − b = a AND c − a = b are both valid checks.
• Focus on the “Undoing”: Explain that subtraction is “undoing” what the addition did, bringing you back to one of the starting parts.
• Use Visuals: Part-part-whole diagrams or bar models can visually represent how the total (sum) can be broken back down into its parts using subtraction.
• Make it a Regular Step: Encourage it as a final step after completing addition problems, especially more complex ones.