Topics
2. Addition and subtraction
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AustraliaNSW
Stage 2

2.01 Estimation first

Lesson
Practice
Lesson

Are you ready?

Do you remember how to  round numbers  ?

Examples

Example 1

The number 379 is plotted on the number line.

050100150200250300350400

Round 379 to the nearest 100 and plot it on a number line.

Worked Solution
Create a strategy

Choose the value of hundreds which is closer to the given number on the number line.

Apply the idea

The number 379 is in between 300 and 400. It is closer to 400, so when you round 379 to the nearest hundreds it would be 400.

050100150200250300350400
Idea summary

To round a number to the nearest 100, plot it on a number line and choose the multiple of 100 that it is closest to.

Estimation first

Let's see how we can estimate the answer to a subtraction problem, and an addition problem.

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Examples

Example 2

We want to estimate the answer to 258-26.

a

Round each of the two numbers to the nearest and then subtract them.

Worked Solution
Create a strategy

Use a number line to round each number to the nearest ten.

Apply the idea

Looking at the number line below, 258 is closer to 260 than to 250.

250251252253254255256257258259260

So 258 rounded to the nearest ten is 260.

Looking at the number line below, 26 is closer to 30 than to 20.

2021222324252627282930

So 26 rounded to the nearest ten is 30.

To find 260-30 we can plot 260 on a number line and jump back by 30:

220230240250260270

260 - 30 = 230

b

Find the value of 258-26.

Worked Solution
Create a strategy

Use a number line to subtract.

Apply the idea

Start at 258 and jump back 26 units.

230235240245250255260

258-26=232

Reflect and check

We can see that the answers are very similar, so the estimation of 230 was a good approximation for the actual answer of 232.

Idea summary

If we don't need an exact answer to a problem, we can use estimating to find out a close-enough answer. We can also use estimating to check our answers, so it is a great skill to learn.

Outcomes

MA2-5NA

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

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