Addition and Subtraction

Checking Your Work

In the last few lessons, you learned how to solve addition and subtraction problems. As you practice these math skills, it's a good idea to get into the habit of checking your work. Checking will help you know if your answers are correct. When you're ready to check the answer to subtraction problems, you'll need to use addition.

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  Let's look at this problem: 9 - 7 = 2.

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  How do we know that 2 is the correct answer? We can check by adding.

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  Let's set up our addition problem. First, we'll write the subtraction problem's answer. That means we'll write 2.

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  Next, we'll add the amount that was subtracted, 7.

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  Time to add. 2 + 7 = 9.

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  If we subtracted correctly, the answer will match the larger number in our subtraction problem.

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  They match — 9 and 9. Our answer was correct.

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  Let's try using addition to check the answer to another subtraction problem: 54 - 21 = 33.

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  Let's set up our addition problem. First write the answer to the subtraction problem, 33.

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  Then add back the number that was subtracted, 21.

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  Now it's time to add. 33 + 21 = 54.

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  Finally, we'll check to see if 54 matches the larger number in our subtraction problem. It does!

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