Topics
3. Multiplication strategies
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Australia
Year 4

3.03 x2, x4

Lesson
Practice
Lesson

Are you ready?

Multiplication looks at equally sized groups. Doubling means multiplying equally sized groups by 2. Can you solve this problem?

Examples

Example 1

Find the value of 2\times8.

Worked Solution
Create a strategy

Multiplying by 2 means to double the number.

Apply the idea
\displaystyle 2\times8\displaystyle =\displaystyle 16Double the number 8
Reflect and check

2 \times 8 can be thought of as 2 lots of 8 stars.

2 circles with 8 stars in each circle.

In the image above there are 16 stars altogether. And 16 is double 8.

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 4

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

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Examples

Example 2

Find the product of 4\times 8.

Worked Solution
Create a strategy

This can be thought of as 4 groups of 8 squares as shown below.

The image shows 4 rows of 8 squares.
Apply the idea

In the above image there are 32 squares in total.4 \times 8 = 32

Reflect and check

We could also use the double, double strategy by doubling the number and then doubling the result:

\displaystyle 2\times 8\displaystyle =\displaystyle 16Double 8
\displaystyle 2\times 16\displaystyle =\displaystyle 32Double the answer
Idea summary

When multiplying by 2 and 4:

  • Double means "multiply by 2."
  • To multiply a number by 4, you can use the Double, Double strategy. That means, double the number, then double the answer.

Outcomes

AC9M4A02

recall and demonstrate proficiency with multiplication facts up to 10 x 10 and related division facts; extend and apply facts to develop efficient mental strategies for computation with larger numbers without a calculator

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