Lesson 96: Checking our work

🧮 MATH (40 Lessons) • 🔵 B. Addition and Subtraction

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Objective

I can use simple strategies to check my answers for addition and subtraction. I can use counters, a number line, or an opposite operation to see if my answer makes sense.

Materials

Mini-lesson — What does it mean to check our work?

Good mathematicians do not just find an answer — they check their work to see if the answer is correct and makes sense.

Strategy 1: Use counters or a picture

  • Build the problem with counters or draw a quick picture.
  • Count again carefully to see if your answer matches.
  • Example: For 6 + 3, make one group of 6 and one group of 3, then count all. You should get 9.

Strategy 2: Use the opposite operation

  • Addition and subtraction are opposites.
  • If you know 7 + 5 = 12, you can check with 12 − 5 = 7.
  • If your subtraction gives you back the starting number, your answer is likely correct.

Strategy 3: Use a number line

  • For addition, start at the first number and jump right.
  • For subtraction, start at the big number and jump left.
  • Check that your jumps match the problem and land on your answer.

Strategy 4: Ask, "Does this make sense?"

  • Think about the story. Could the answer be that big or that small?
  • If you start with 10 stickers and give 2 away, could you have 20 left? (No!)

In this lesson, we will solve problems and then use at least one strategy to check each answer.

Picture strip: Check with the opposite operation

Fact pair: 7 + 3 and 10 − 3

Check steps

Guided Practice — Solve, then check

You will solve a few problems and then use one or two strategies to check each answer.

  1. Choose a problem: Pick an easy addition or subtraction problem within 20, such as 8 + 4 or 15 − 6.
  2. Solve it once: Use counters, a picture, or mental math to find your answer.
  3. Check with counters: Build the problem with counters and count again. Does it match your answer?
  4. Check with the opposite operation: If you solved 8 + 4 = 12, check with 12 − 4 or 12 − 8. Do you get back the starting number?
  5. Use a number line: Draw a small number line. For addition, jump right; for subtraction, jump left. Make sure your jumps match the problem and the answer.
  6. Ask, "Does it make sense?" Think about the story. If a story starts with 10, your answer should not be 50. If it seems too big or too small, check again.
  7. Use the tracing pad: Trace words like check, again, add, subtract, left, or numbers such as 10, 12, 15, or 20. Then write one short sentence to explain how you checked your work.
Tracing Pad
Tracing snapshot for print
Tip: After you trace your words, write a short sentence such as "I used subtraction to check my addition." Draw a tiny number line or counters to match.

Practice 1 — Check addition problems

[PLACEHOLDER PRACTICE 1]

Practice 2 — Check subtraction problems

[PLACEHOLDER PRACTICE 2]

Practice 3 — Check real-life stories

[PLACEHOLDER PRACTICE 3]

Quick Check — Checking our work

Answer each question about checking your work. You may use counters or a number line to help you.

1) What does it mean to check your work in math?

Checking means looking again to see if your answer is correct and makes sense.

2) Which tool can help you check an addition problem?

Counters and number lines show the math so you can check it.

3) "You think 6 + 5 = 10." How could you check this quickly?

Build 6 and 5 with counters and count to see that 6 + 5 is actually 11, not 10.

4) Which pair of equations shows opposites you could use to check?

8 + 4 and 12 − 4 are opposite operations that check each other.

5) You solved 9 − 3 = 7. How can you use addition to check?

If the subtraction were correct, 7 + 3 would give 9. Because 7 + 3 = 10, you should check again.

6) On a number line, how do you check a subtraction like 12 − 4?

Subtraction on a number line is shown by jumping left from the big number.

7) You think 7 + 8 = 15. What is a good check?

If 7 + 8 = 15, then 15 − 8 should give you 7.

8) "There are 10 apples in a bowl. 4 are eaten." Which answer looks like it does not make sense?

You cannot have more apples than you started with after eating some.

9) Which question helps you decide if an answer makes sense?

We ask if the answer could really happen in the story.

10) You solved 14 − 5 = 9. Which addition problem could you use to check?

If 14 − 5 = 9, then 9 + 5 should give you 14.

11) You used a number line to solve 5 + 6. How can you check your jumps?

Checking your jumps helps you see if the number line matches the problem.

12) Which of these is a good habit when you finish a math problem?

Good mathematicians look back and check their work.

13) You solved 3 + 9 = 11. You check with counters and see 3 + 9 = 12. What should you do?

Checking showed a mistake, so you fix the answer to 12.

14) Which pair of equations checks each other?

10 − 2 and 8 + 2 are opposite operations that match and check each other.

15) Why is it helpful to learn to check your work?

Checking helps you find mistakes and feel confident in your math.

Assessment (parent/teacher)

Exit ticket (student)

Next time I will practise…

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