Topics
3. Multiplication strategies
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Australia
Year 4

3.04 x8

Lesson
Practice
Lesson

Are you ready?

Multiplication looks at equally sized groups.  Doubling  means multiplying equally sized groups by 2. Let's try this problem to remember how.

Examples

Example 1

Find the value of 9\times2.

Worked Solution
Create a strategy

Remember 9 \times 2 is the same as 2 \times 9. This can be thought of as 2 groups of 9.

The picture below shows 2 groups of 9 stars.

The image shows 2 circles with 9 stars inside each circle.
Apply the idea

In the above image there are 18 stars altogether.

\displaystyle 9\times2\displaystyle =\displaystyle 18
Idea summary

Multiplying by two is the same as doubling and can be thought of as the total of two equally sized groups.

Multiply numbers by 8

4 groups is twice (two times) as many as 2 groups. 8 groups is twice (two times) as many as 4 groups. Let's look at how to multiply numbers by 8 using a Double, Double, Double strategy.

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Examples

Example 2

Find the value of 8 \times 5.

Worked Solution
Create a strategy

This can be thought of as 8 groups of 5 squares, as represented in the image below.

The image shows eight rows of five squares.
Apply the idea

There are 40 squares in the above image.8 \times 5 = 40

Reflect and check

Or we can use the double, double, double strategy on 5 to get:

\displaystyle 5 \times 2\displaystyle =\displaystyle 10Double 5
\displaystyle 10 \times 2\displaystyle =\displaystyle 20Double the answer
\displaystyle 20 \times 2\displaystyle =\displaystyle 40Double the answer again

Which means: 8 \times 5 = 40

Idea summary

You can use the Double, Double, Double strategy to multiply a number by 8. To do this, double the number, double the answer, then double the answer again.

Outcomes

ACMNA074

Investigate number sequences involving multiples of 3, 4, 6, 7, 8, and 9

ACMNA075

Recall multiplication facts up to 10 × 10 and related division facts

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