Question

Easy Rules for Divisibility please tell me

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The Divisibility Rules

These rules let you test if one number is divisible by another, without having to do too much calculation!

Example: is 723 divisible by 3?

We could try dividing 723 by 3

Or use the "3" rule: 7+2+3=12, and 12 ÷ 3 = 4 exactly  Yes

Note: 0 (zero) is a "yes" result to any of these tests.

 

1

Any integer (not a fraction) is divisible by 1

2

The last digit is even (0,2,4,6,8)

128  Yes

129  No

3

The sum of the digits is divisible by 3

381 (3+8+1=12, and 12÷3 = 4) Yes

217 (2+1+7=10, and 10÷3 = 3 1/3) No

This rule can be repeated when needed:

99996 (9+9+9+9+6 = 42, then 4+2=6) Yes

4

The last 2 digits are divisible by 4

1312 is (12÷4=3) Yes

7019 is not (19÷4=4 3/4) No

A quick check (useful for small numbers) is to halve the number twice and the result is still a whole number.

12/2 = 6, 6/2 = 3, 3 is a whole number. Yes

30/2 = 15, 15/2 = 7.5 which is not a whole number. No

5

The last digit is 0 or 5

175  Yes

809  No

6

Is even and is divisible by 3 (it passes both the 2 rule and 3 rule above)

114 (it is even, and 1+1+4=6 and 6÷3 = 2) Yes

308 (it is even, but 3+0+8=11 and 11÷3 = 3 2/3) No

7

Double the last digit and subtract it from a number made by the other digits. The result must be divisible by 7. (We can apply this rule to that answer again)

672 (Double 2 is 4, 67-4=63, and 63÷7=9) Yes

105 (Double 5 is 10, 10-10=0, and 0 is divisible by 7) Yes

905 (Double 5 is 10, 90-10=80, and 80÷7=11 3/7) No

8

The last three digits are divisible by 8

109816 (816÷8=102) Yes

216302 (302÷8=37 3/4) No

A quick check is to halve three times and the result is still a whole number:

816/2 = 408, 408/2 = 204, 204/2 = 102 Yes

302/2 = 151, 151/2 = 75.5 No

9

The sum of the digits is divisible by 9

(Note: This rule can be repeated when needed)

1629 (1+6+2+9=18, and again, 1+8=9) Yes

2013 (2+0+1+3=6) No

10

The number ends in 0

220  Yes

221  No

11

Add and subtract digits in an alternating pattern (add digit, subtract next digit, add next digit, etc). Then check if that answer is divisible by 11.

1364 (+1−3+6−4 = 0) Yes

913 (+9−1+3 = 11) Yes

3729 (+3−7+2−9 = −11) Yes

987 (+9−8+7 = 8) No

12

The number is divisible by both 3 and 4 (it passes both the 3 rule and 4 rule above)

648 
(By 3? 6+4+8=18 and 18÷3=6 Yes) 
(By 4? 48÷4=12 Yes) 
Both pass, so Yes

524 
(By 3? 5+2+4=11, 11÷3= 3 2/3 No) 
(Don't need to check by 4) No

There are lots more! Not only are there divisibility tests for larger numbers, but there are more tests for the numbers we have shown.

Factors Can Be Useful

Factors are the numbers you multiply to get another number:



This can be useful, because:

When a number is divisible by another number ...

... then it is also divisible by each of the factors of that number.