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What is Checking Subtraction Using Addition?
Grade Level:
Class 3
Maths, Computing
Definition
What is it?
Checking subtraction using addition means using addition to make sure your subtraction answer is correct. It's like doing a reverse check to confirm your maths. If your addition check works out, then your subtraction was right!
Simple Example
Quick Example
Imagine you have 10 laddoos and you eat 3. You're left with 7 laddoos (10 - 3 = 7). To check this, you can add the laddoos you ate (3) to the laddoos you have left (7). If 3 + 7 equals your original 10 laddoos, then your subtraction is correct!
Worked Example
Step-by-Step
Let's subtract 45 from 87 and then check our answer using addition.
Step 1: Perform the subtraction.
87 - 45 = ?
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Step 2: Subtract the ones digits: 7 - 5 = 2.
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Step 3: Subtract the tens digits: 8 - 4 = 4.
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Step 4: So, 87 - 45 = 42. This is our subtraction answer.
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Step 5: Now, let's check using addition. We add our answer (42) to the number we subtracted (45).
42 + 45 = ?
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Step 6: Add the ones digits: 2 + 5 = 7.
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Step 7: Add the tens digits: 4 + 4 = 8.
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Step 8: The sum is 87. Since 87 is the same as our original starting number, our subtraction was correct!
Answer: The subtraction 87 - 45 = 42 is correct because 42 + 45 = 87.
Why It Matters
Learning to check your work builds strong problem-solving skills, which are super useful in computing and data analysis. This skill helps engineers double-check calculations for building bridges or software developers ensuring their code works perfectly. It teaches you to be thorough and precise.
Common Mistakes
MISTAKE: Adding the original larger number to the answer. For example, if 10 - 3 = 7, checking by doing 10 + 7. | CORRECTION: Always add the answer (difference) to the number that was subtracted (subtrahend). So, 7 + 3 should equal 10.
MISTAKE: Not understanding which numbers to use in the check. Students might get confused between the 'minuend', 'subtrahend', and 'difference'. | CORRECTION: Remember the formula: Difference + Subtrahend = Minuend. (Answer + Number Subtracted = Starting Number).
MISTAKE: Making a mistake in the addition check itself, leading to a wrong conclusion about the subtraction. | CORRECTION: Be careful and double-check your addition step too! The check only works if the addition is done correctly.
Practice Questions
Try It Yourself
QUESTION: Subtract 23 from 56. Then, use addition to check your answer. | ANSWER: 56 - 23 = 33. Check: 33 + 23 = 56. The answer is correct.
QUESTION: A bus started with 75 passengers. At a stop, 18 passengers got off. How many passengers are left? Check your answer using addition. | ANSWER: 75 - 18 = 57 passengers. Check: 57 + 18 = 75. The answer is correct.
QUESTION: Rohan had Rs 125. He bought a notebook for Rs 35 and a pen for Rs 15. How much money does he have left? Check your final subtraction using addition. | ANSWER: First, total spent = 35 + 15 = Rs 50. Money left = 125 - 50 = Rs 75. Check: 75 + 50 = 125. The answer is correct.
MCQ
Quick Quiz
If 90 - 30 = 60, which addition check proves this subtraction is correct?
90 + 60 = 150
60 + 30 = 90
90 + 30 = 120
60 - 30 = 30
The Correct Answer Is:
B
Option B is correct because to check subtraction, you add the answer (60) to the number you subtracted (30). This should give you the original starting number (90).
Real World Connection
In the Real World
This checking method is used daily by shopkeepers in local kirana stores when they calculate change for customers. They subtract the bill amount from the money given, then mentally add the change back to the bill to ensure the total matches what the customer paid. It ensures no mistakes are made!
Key Vocabulary
Key Terms
SUBTRACTION: Taking one number away from another to find the difference | ADDITION: Combining two or more numbers to find their total sum | DIFFERENCE: The result you get after subtracting one number from another | MINUEND: The number from which another number is subtracted (the starting number) | SUBTRAHEND: The number that is subtracted from the minuend
What's Next
What to Learn Next
Great job learning to check your subtractions! Next, you can explore 'Inverse Operations: Addition and Subtraction'. This will show you how addition and subtraction are opposite actions that undo each other, which is exactly why our checking method works so well!
