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PRACTICE: x2,4,8


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Can you solve this multiplication by  doubling  ?


Example 1

Double 9.

Worked Solution
Create a strategy

"Double" means "multiply by 2."

Apply the idea

We can think of 9\times 2 as 2 groups of 9 just like in the image below:

The image shows 2 rows of 9 squares.

Count the number of squares to get:

9\times 2 =18

Idea summary

To double a number, we multiply the number by 2.

Multiply by 2, 4, and 8

Let's look at the relationship between the 2, 4, and 8 times tables.

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Example 2

Find 6\times 8.

Worked Solution
Create a strategy

We can think of this as 6 groups of 8. The picture shows 6 groups of 8 squares.

An array of squares with 6 rows and 8 columns.
Apply the idea

There are 48 squares in the array.6\times8=48

Reflect and check

We can also use the double, double, double strategy on 6 to get:

\displaystyle 6 \times 2\displaystyle =\displaystyle 12Double 6
\displaystyle 12 \times 2\displaystyle =\displaystyle 24Double the answer
\displaystyle 24 \times 2\displaystyle =\displaystyle 48Double the answer again

Which means: 6 \times 8 = 48

Idea summary
  • \times \,2 means "double the number."

  • \times \,4 means "double the number, then double the answer."

  • \times \,8 means "double the number, double the answer, then double the answer again."



uses mental and informal written strategies for multiplication and division

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