Topics
5. Addition and subtraction
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Australia
Year 3

5.02 Addition and subtraction

Lesson
Practice
Lesson

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Using a number line is helpful to solve  addition  or  subtraction  problems, and helps when we have more than one part to our problem.

Examples

Example 1

Use the number line to help you find the value of 43+12.

40455055606570
Worked Solution
Create a strategy

Jump 12 steps to the right of 43 on the number line.

Apply the idea
40455055606570

By jumping to the right 12 steps we end up at the number 55. So:43+12=55

Idea summary

To add numbers on the number line, find the bigger number on the number line then jump to the right by the number we are adding.

Add and subtract story and word problems

Sometimes we need to read written or story problems, and work out what we need to solve. In this video, we have to find out what our problem is, then solve it.

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Examples

Example 2

There were 14 flowers in Eileen’s garden. 3 more flowers grew, then Eileen picked 6 flowers to put in a vase.

a

Which number sentence matches the word problem for the number of flowers left in the garden?

A
14-3+6
B
14+3-6
Worked Solution
Create a strategy

The word "more" and "grew" mean addition because the number of flowers got bigger. The word "picked" means subtraction because the flowers were taken away.

Apply the idea

The number of flowers left in the garden matches the number sentence: 14+3-6 So the answer is option B.

b

How many flowers are left in the garden?

Worked Solution
Create a strategy

Use the number sentence in part (a) to find the number of flowers left in the garden.

Apply the idea
\displaystyle \text{No. of flowers left}\displaystyle =\displaystyle 14+3-6Write the number sentence in part (a)
\displaystyle =\displaystyle 17-6Add 14 and 3
\displaystyle =\displaystyle 11On a number line count back 6 from 17
02468101214161820
Idea summary

When we have a word problem, we need to think about the story and use the keywords to work out which operators to use. Some words that we may find in a story could be:

additionsubtraction
moreless
addsubtract
give totake away
plusminus

Outcomes

AC9M3A02

extend and apply knowledge of addition and subtraction facts to 20 to develop efficient mental strategies for computation with larger numbers without a calculator

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