New
New
Year 3

Add two 3-digit numbers using adjusting strategies

I can adjust a number to make an addition easier.

New
New
Year 3

Add two 3-digit numbers using adjusting strategies

I can adjust a number to make an addition easier.

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Lesson details

Key learning points

  1. If you adjust an addend you adjust the sum by the inverse.
  2. If you add 1 to an addend you must subtract 1 from the sum.
  3. If you subtract 1 from an addend you must add 1 to the sum.
  4. A number line can represent the adjusting strategy.

Common misconception

Many pupils will want to use a written method/algorithm immediately such as column method. This lesson is to support mental addition strategies.

Encourage the children to forget about using written procedures because they are inefficient when the addends are suited to adjustment. As class teacher, use adjustment and race the children desperate to use columns to see who is fastest!

Keywords

  • Adjust - When you adjust you make a small change to a number. This is done to make a calculation easier to solve mentally.

  • Inverse - The inverse is the opposite or reverse operation. For example, subtraction is the inverse operation of addition.

  • Efficient - Working efficiently means finding a way to solve a problem quickly whilst also maintaining accuracy.

This strategy is shown with practical resources and jottings but the end goal is for the children be able to do it mentally so try to reduce such supports as quickly as possible.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
Which of these numbers is closest to 10?
12
13
Correct answer: 9
6
15
Q2.
Which of these numbers is closest to a tens boundary?
54
48
53
46
Correct answer: 51
Q3.
Which of these numbers is closest to 100?
89
110
Correct answer: 98
107
103
Q4.
Which number is closest to a hundreds boundary?
514
505
491
Correct answer: 501
502
Q5.
Which of these number lines accurately represents 22 + 12 = 34 using partitioning to bridge?
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q6.
Match the complements to 100
Correct Answer:92,8

8

Correct Answer:95,5

5

Correct Answer:89,11

11

Correct Answer:98,2

2

Correct Answer:75,25

25

Correct Answer:60,40

40

6 Questions

Q1.
What is the inverse of subtract 4?
Add 5
Correct answer: Add 4
Subtract 4
Multiply by 4
Divide by 4
Q2.
Which of these number line represents using adjustment to find the total of 25 + 49?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
Q3.
Solve the following sum using adjustment. 360 + 499 =
Correct Answer: 859, eight hundred and fifty nine
Q4.
Solve the following sum using adjustment. = 390 + 270
Correct Answer: 660, six hundred and sixty
Q5.
Solve the following sum by adjusting both addends. = 199 + 599
Correct Answer: 798, seven hundred and ninety eight
Q6.
Solve the following sum using adjustment. 397 + 296 =
Correct Answer: 693, six hundred and ninety three