Year 3 - Maths
Add two 3-digit numbers using adjusting strategies
Informal and mental strategies for adding and subtracting two 3-digit numbers
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This pupil-friendly study guide includes a unit summary, clear notes, common mistakes, and quick questions with answers.
Unit Summary
In this unit pupils will add and subtract two 3-digit numbers using partitioning, adjusting, redistribution and bridging.
Lesson Summary
You will learn to adjust a number to make an addition easier.
Key Notes
- If you adjust an addend you adjust the sum by the inverse.
- If you add 1 to an addend you must subtract 1 from the sum.
- If you subtract 1 from an addend you must add 1 to the sum.
- A number line can represent the adjusting strategy.
Vocabulary To Learn
- 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.
Common Mistakes To Avoid
- Many pupils will want to use a written method/algorithm immediately such as column method. This lesson is to support mental addition strategies.
3 Quick Questions (With Answers)
1. Solve this in steps: explain the method from this lesson and why each step matters.
If you adjust an addend you adjust the sum by the inverse. If you add 1 to an addend you must subtract 1 from the sum.
2. Use this key word in a maths sentence and explain it clearly. 'Adjust'
When you adjust you make a small change to a number. This is done to make a calculation easier to solve mentally. Use this word when showing your method, then check your final answer is reasonable.
3. Spot and fix this common maths mistake.
Mistake: Many pupils will want to use a written method/algorithm immediately such as column method. This lesson is to support mental addition strategies. Correction: 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!
More Lessons In This Unit
- Add 2 and 3-digit numbers by redistributing Study Guide
- Add two 3-digit numbers using partitioning Study Guide
- Choose the most efficient strategy to add two 3-digit numbers Study Guide
- Choose the most efficient strategy to subtract from a 3-digit number Study Guide
- Subtract 2 or 3-digit numbers using partitioning and bridging a multiple of 10 Study Guide
- Subtract 3-digit multiples of 10 by finding the difference between them Study Guide
- Subtract a pair of 2-digit numbers by finding the difference Study Guide
- Use addition and subtraction to solve problems in different contexts Study Guide
- Use addition and subtraction to solve problems involving bar charts, pictograms and tables Study Guide
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