Topics
4. Subtraction
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AustraliaNSW
Stage 2

4.03 Estimation first

Lesson
Practice
Lesson

Are you ready?

Do you remember how to  estimate the answer to an addition problem  ?

Examples

Example 1

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 correct 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.

Estimation for subtraction

Let's see how we can estimate the answer to a subtraction problem, by rounding our numbers first.

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Examples

Example 2

Choose the right answer.

46 - 2 will be in the:

A
40s
B
30s
Worked Solution
Create a strategy

Using the number line, jump 2 times to the left from 46.

Apply the idea

Locate where 46 is on the number line, and jump to the left by 2 places.

414243444546474849

By jumping to the left by 2, we are still in the 40s, so the correct answer is option A.

Idea summary

When we need to solve a subtraction problem, we can still round our numbers to the nearest 10,\,100, or 1000, just as we would for addition. We don't need the exact answer so this gives us an idea of what our answer should be close to.

Outcomes

MA2-5NA

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

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