Lesson

Ideas

Let's try this problem to help us remember how to double a number.

Examples

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:

Count the number of squares to get:

9\times 2 =18

Idea summary

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

When we add the same number more than once, we can think of it as multiplication, as we see in this video.

Examples

Example 2

Write this addition as a multiplication. 4+4+4+4

Worked Solution
Create a strategy

We can rewrite the addition as a multiplication using: \text{Number of groups}\times\text{Amount in each group}.

Apply the idea

We have 4 groups of 4. We can rewrite this as multiplication: 4+4+4+4=4\times 4

Idea summary

If we are adding groups of the same size, we can also write it as a multiplication:

\text{Number of groups}\times\text{Amount in each group}

Outcomes

MA2-5NA

uses mental and written strategies for addition and subtraction involving two-, three-, four and five-digit numbers