Topics
3. Addition
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AustraliaNSW
Stage 2

3.04 Estimation first

Lesson
Practice
Lesson

Are you ready?

Do you remember how to  round numbers  ? Give this problem a try.

Examples

Example 1

Round 54 to the nearest ten.

Worked Solution
Create a strategy

Find which tens the number is closest to using number lines.

Apply the idea

Looking at the number line below, 54 is closer to 50 than to 60.

5051525354555657585960

So 54 rounded to the nearest ten is 50.

Idea summary

We can use number lines to help when rounding numbers. By putting the number in our question on a number line, we can see the closest number.

Estimation first addition

Let's see how we can estimate the answer to an addition problem by rounding the numbers first.

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Examples

Example 2

Choose the right answer.

143 + 33 will be in the:

A
100s
B
200s
Worked Solution
Create a strategy

Round each number to the nearest ten using a number line then use estimation.

Apply the idea

143 is plotted on the number line below:

140141142143144145146147148149150

We can see it is closer to 140 than 150.

33 is plotted on the number line below:

3031323334353637383940

We can see it is closer to 30 than 40.

So we can estimate the value of 143+33 by finding the value of 140+30 on the number line.

130140150160170180

By jumping to the right by 30 we get an estimation of 170. Since this number is in the 100s, the answer is option A.

Idea summary

Once we round the numbers in our question, we can then add them together to get an estimate of our answer.

Outcomes

MA2-5NA

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

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