3. Multiplication strategies

3.01 Repeated addition as multiplication

3.02 Groups and arrays

PRACTICE: x2,4,8

PRACTICE: Multiplication

3.10 Multiply and divide by 10; the powers of 10

Year 4

3.06 x6

Lesson

Ideas

Are you ready?
Multiply numbers by 6

Are you ready?

Multiplication looks at equally sized groups. Can you solve this problem?

Examples

Example 1

Find the value of 3\times 9.

Worked Solution

Create a strategy

We can think of 3 \times 9 as 3 groups of 9 like in the image below that has 3 groups of 9 squares:

The image shows 3 rows of 9 squares.

Apply the idea

The above image has 27 squares in total.

\displaystyle 3\times9

\displaystyle =

\displaystyle 27

Idea summary

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

Multiply numbers by 6

Let's look at how to multiply numbers by 6.

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Examples

Example 2

Find the value of 6\times3.

Worked Solution

Create a strategy

We can think of 6\times 3 as 6 groups of 3 just like in the image below that has 6 groups of 3 squares:

The image shows 6 rows of 3 squares.

Apply the idea

The above image has 18 squares in total.

6 \times 3 = 18

Reflect and check

We can also multiply the number by 3 then double the answer:

\displaystyle 3\times 3	\displaystyle =	\displaystyle 9	Multiply by 3
\displaystyle 2\times 9	\displaystyle =	\displaystyle 18	Double the answer

Idea summary

You can use a doubling strategy to multiply a number by 6. Start by multiplying the number by 3, 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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