4. Subtraction

4.01 Subtraction and number lines

4.02 Counting back

4.03 Estimation first

4.04 Subtraction strategies

4.05 Subtraction using algorithm

4.06 Subtraction with regrouping

PRACTICE: Subtraction

Year 3

4.04 Subtraction strategies

Lesson

Ideas

Are you ready?
Subtraction strategies
Subtraction strategies compensation

Are you ready?

Can you remember how we use a number line to subtract one number from another? Give this problem a try.

Examples

Example 1

Use the number line to help you find the value of 46 - 12.

Worked Solution

Create a strategy

Locate the bigger number on the number line and jumpt to the left by the smaller number.

Apply the idea

Locate where 46 is on the number line. We can find 46-12 by jumping from 46 to the left by 12:

We have ended up on the number 34. So:46 - 12 = 34

Idea summary

Number lines can help us visualise subtraction using jumps.

Subtraction strategies

We can use place value models as well as the bridge to ten strategy to help us with subtraction. Let's see how in this video.

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Examples

Example 2

Use a strategy to find 36 - 7.

Worked Solution

Create a strategy

Use the bridge to 10 strategy.

Apply the idea

We need to take 6 away from 36 to make 30. This leaves us with 1 left to take away.

Using a number line, we can find 30-1 to get 29:

This means we have:

\displaystyle 36 - 7	\displaystyle =	\displaystyle 30 - 1	Subtract 6 from 36
	\displaystyle =	\displaystyle 29	Subtract 1 from 30

Idea summary

Bridging to 10 can help us get to 10, so that subtracting can be much quicker.

Subtraction strategies compensation

Another way to solve subtraction is to use the compensation strategy. In this video we can see just how it works.

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Examples

Example 3

If 90 - 13 = 77, use a strategy to find 89 - 13.

Worked Solution

Create a strategy

Use the compensation strategy.

Apply the idea

If we look at 90-13 and 89 - 13, we can see that 90 has been changed to 89.

89 is 1 less than 90, so we need to subtract 1 from 90 - 13 = 77. To subtract 1 from 77 we can use a number line and find 77 and move 1 place to the left to get 76:

This means that: 89-13 = 76

Idea summary

Compensation uses the answer of one problem, to find the answer to another problem

Outcomes

AC9M3A02

extend and apply knowledge of addition and subtraction facts to 20 to develop efficient mental strategies for computation with larger numbers without a calculator

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