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

4.01 Subtraction and number lines

Lesson

Ideas

Are you ready?
Subtraction and number lines

Are you ready?

Knowing how to count back can help us with subtracting on a number line.

Examples

Example 1

Create a pattern by subtracting 1 each time.

6,\,⬚,\,⬚,\,⬚

Worked Solution

Create a strategy

We can use a number line to find how much the numbers are decreasing by each time.

Apply the idea

Plot 6 on a number line and jump back by 1 to get to 5:

Now we complete the pattern by subtracting 1 each time:

\displaystyle 5-1	\displaystyle =	\displaystyle 4	Subtract 1
\displaystyle 4-1	\displaystyle =	\displaystyle 3	Subtract 1

So the complete pattern is: 6,\, 5,\, 4,\, 3.

Subtraction and number lines

We are going to use a number line to see how to find the difference between two numbers.

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Examples

Example 2

Use the number line to help you find the value of 35 - 24.

Worked Solution

Create a strategy

Plot the first number on a number line then jump back the amount of the second number.

Apply the idea

Plot 35 on the number line and jump back by 24 to get 11:

35 -24 = 11

Idea summary

When adding to a number, jump forward (to the right).

A number line with 4 curved arrows showing jumps to the right.

When subtracting from a number, jump backwards (to the left).

A number line with 4 curved arrows showing jumps to the left.

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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