# Check the Order of Magnitude by Rounding

In this worksheet, students check the order of magnitude of a calculation by rounding first.

Key stage:  KS 3

Curriculum topic:   Number

Curriculum subtopic:   Use Calculators/Technology for Accuracy

Difficulty level:

#### Worksheet Overview

When you use a calculator, you must have a rough idea of the expected size of your answer.

This size is called the order of magnitude and you can get a good idea by rounding off the question first.

7840 × 28 is approximately 8000 × 30 which is 240000

24216 - 18947 is approximately 24000 - 19000 = 5000

542 ÷ 18 is approximately 500 ÷ 20 or 25

Example

Let's find the order of magnitude for 3.42  × 19.

3.42 × 19 lies between
 A
and
 B
which equal
 C
and
 D

3.42 × 19 lies between 3 × 19 and 4 × 19.

3.42 × 19 lies between
 3 × 19
and
 4 × 19
which equal
 57
and
 76

Check to see how close you are by doing the calculation on your calculator as well.

3.42 × 19 =
 64.98

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