  # Divisibility Rules (1)

In this worksheet, students show their understanding of the rule for divisibility by 11. Key stage:  KS 3

Curriculum topic:   Number

Curriculum subtopic:   Use Concepts and Vocabulary for All Numbers

Difficulty level:   #### Worksheet Overview

Instead of actually carrying out a division, it is possible to "test" a number to see if it can be divided exactly by certain numbers.

If a number divides exactly into a number, it is divisible by this number.

24 is divisible by 2, because 24 ÷ 2 is a whole number.

24 is not divisible by 5, because 24 ÷ 5 is not a whole number.

Here is the rule to test divisibility by 11.

Rule for 11

To test a number for divisibility by 11 we must find the sums of alternative digits.  This is best illustrated using examples.

Example 1

Determine whether 17281 is divisible by 11.

Method

Underline alternate numbers to get 17226

Add up the underlined digits to get the sum of 1 + 2 + 6 = 9

Add up the non-underlined digits to get the sum of 7 + 2 = 9

If these two digit sums are equal or differ by a multiple of 11, the original number is divisible by 11.

Here they are equal so 17226 is divisible by 11.

Example 2

Determine whether 17281357 is divisible by 11.

Method

Underline alternate numbers to get 17281539

Add up the underlined digits to get the sum of 1 + 2 + 1 + 3 = 7

Add up the non-underlined digits to get the sum of 7 + 8 + 5 + 9 = 29

If these two digit sums are equal or differ by a multiple of 11, the original number is divisible by 11.

Here they differ by 29 - 7 = 22 so 17281539 is divisible by 11.

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