# Use Knowledge of Numbers to Estimate Answers and Check Using the Inverse Operation.

In this worksheet, students will estimate the answer to addition and subtraction questions using their knowledge of number. They will also use the inverse operation to check answers.

Key stage:  KS 2

Curriculum topic:   Number: Addition and Subtraction

Curriculum subtopic:   Estimate and Check Answers

Difficulty level:

#### Worksheet Overview

It can be useful to estimate the answer to a question.

We often do this to see if the actual answer is likely to be correct -  the estimation should be quite close to the answer!

We can use rounding to help us make a good estimation of an answer.

Some numbers need to be rounded up and others need to be rounded down.

In this activity, we are going to use our knowledge of the number system to make estimations rounding to a multiple of 5 or 10.

Have a look at the following example.

Example 1

652 + 326

We can see that 652 is close to 650

We can see that 326 is close to 325

These numbers are quite simple to add mentally (or on paper if we need to).

650 + 325 = 975

The actual answer is 978  (975 was a really good estimate).

We can check our answers using the inverse operation:  978 - 326 =  652

Example 2

83 - 31

We can see that 83 is close to 80

We can see that 31 is close to 30

Now subtract: 80 - 30 = 50

The actual answer is 83 - 31 = 52 (another  good estimate).

Rounding the number to the nearest multiple of 5, 10 or even 100 can help us make a good estimate.

We can check our answers using the inverse operation:  52 + 31 = 83

Are you ready to try some questions now?

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