Topics
4. Subtraction
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Australia
Year 3

4.02 Counting back

Lesson
Practice
Lesson

Are you ready?

Do you remember how to  count forward  from any number?

Examples

Example 1

Complete the pattern by adding 2 each time.

40,42,44, ⬚, ⬚, ⬚

Worked Solution
Create a strategy

Imagine the numbers are written in the place value table. Add the values in each column.

TensOnes
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Apply the idea
\displaystyle 44 + 2\displaystyle =\displaystyle 46Add 2
\displaystyle 46 + 2\displaystyle =\displaystyle 48Add 2
\displaystyle 48 + 2\displaystyle =\displaystyle 50Add 2

The complete pattern is 40,\,42,\,44,\,46,\,48,\,50.

Idea summary

To count forwards we add the numbers.

Counting back

We can also count backwards from any number. Let's see how we can count backwards by 1,2,5, or 10.

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Examples

Example 2

Find the next number in the pattern:

44,\, 39,\,34,\,29,\,24,\,19,\, ⬚

Worked Solution
Create a strategy

We can see how much the numbers are decreasing by counting down on a number line from one of the numbers to the next number in the pattern.

Apply the idea

Count backwards from 44 to 39 to get the difference of 5:

3536373839404142434445

Now we can subtract 5 from the last number in the pattern to get the next number in the pattern:

\displaystyle 19-5\displaystyle =\displaystyle 14Subtract 5

So the complete pattern is: 44,\, 39,\,34,\,29,\,24,\,19,\, 14.

Idea summary

When we count backwards, our number gets smaller. We can use a number line to help us, by jumping left. Look at how we can count backwards by 2 below.

Number line from 0 to 15 that it counting backwards by 2. Ask your teacher for more information.

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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