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To find if number x is divisible by: 2: If x is even, True 3: If the sum of the digits of x are a multiple of 3, True 4: If the ones and tens digits form a number that is divisible by 4, True 5: If the ones digit is a 0 or 5, True 6: If x is divisible by 2 AND 3, True 7: Double the ones digit and subtract the last number from the remaining digits. If difference divisible by 7, True 8: If last 3 digits form a number that is divisible by 8, True OR If x is divisible by 2 three times, True 9: If the sum of the digits are a multiple of 9, True 10: If the ones digit is 0, True 11 (Method 1): Add each digit using these properties: - + - +... If the resulting number is divisible by 11, True 11 (Method 2): Starting with ones digit, add every other number (A). Add the remaining numbers (B). If A - B is divisible by 11, True 12: If sum of the digits is a multiple of 3 and the last two digits are a multiple of 4, True 15: If x is divisible by 3 AND 5, True

4 Example: Is 312 divisible by 4? If the ones and tens digits form a number that is divisible by 4 then true. The ones and tens digits form 12 and 12 is divisible by 4, therefore true.

7 Example: Is 357 divisible by 7? Double the ones digit (7) to get 14. Subtract 14 from remaining digits (35) to get 21. 21 is divisible by 7, therefore true.

9 Example: Is 95,301 divisible by 9? The number 95,301 is divisible by 9 because the digits add to 18 (9+5+3+0+1), which is a multiple of 9.

11 (Method 1) Example: Is 824,472 divisible by 11? -8 + 2 - 4 + 4 - 7 + 2 = -11, which is divisible by 11, therefore 824,472 is divisible by 11.

11 (Method 2) Example: Is 824,472 divisible by 11? Starting with the units digit, add every other number:2 + 4 + 2 = 8. Then add the remaining numbers: 7 + 4 + 8 = 19. Since the difference between these two sums is 11, which is divisible by 11, 824472 is divisible by 11.

If anyone knows any other divisible number properties, please list them. Thanks.

Last edited by I3igDmsu on 28 Feb 2010, 19:30, edited 15 times in total.

There's no reason to exclude 0. Zero is definitely divisible by 2;

I3igDmsu wrote:

4: If the ones and tens digits of x are divisible by 4, True

You mean: if the tens and ones digits form a number which is divisible by 4. The number 31,212 is divisible by 4, for example, because the last two digits form a number (12) which is divisible by 4, even though the tens digit and the ones digit are not individually divisible by 4.

I3igDmsu wrote:

5: If the ones digit is a 0 or 5, True (excludes x = 0)

There's no reason to exclude 0. Zero is definitely divisible by 5;

I3igDmsu wrote:

8: If last 3 digits are divisible by 8, True OR If x is divisible by 2 three times, True

Again for clarity - the last three digits should form a number divisible by 8. For example, 85,328 is divisible by 8 because 328 is divisible by 8 (328 = 8*41).

I3igDmsu wrote:

9: If the sum of the digits are 9, True

You mean: If the sum of the digits is a multiple of 9. The sum does not need to equal 9. The number 95,301 is divisible by 9, for example, because the digits add to 18, which is a multiple of 9.

I3igDmsu wrote:

10: If the ones digit is 0, True (excludes x=0)

There's no reason to exclude 0. Zero is definitely divisible by 10;
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11 Example: Is 824,472 divisible by 11? Starting with the units digit, add every other number:2 + 4 + 2 = 8. Then add the remaining numbers: 7 + 4 + 8 = 19. Since the difference between these two sums is 11, which is divisible by 11, 824472 is divisible by 11.

If anyone knows any other divisible number properties, please list them. Thanks.

Ok there's maybe one little simplification for the divisibility of number 11:

There's one scheme applied to the digits : - + - + etc. if the forming number is divisible by 11 then our number is divisible by 11.

Lets take for ex 3916 which is divisible by 11. Application : -3 + 9 - 1 + 6 = 11 -----> so 3916 is divisible by 11.

Another example. 1099989

-1+0-9+9-9+8-9 = -11 --------. so 1099989 is divisible by 11.

7: Double the ones digit and subtract the last number from the remaining digits. If difference divisible by 7, True

I cannot for the life of me figure out how this works. Would anybody be kind enough to break this methodology down with a specific example so I can see what I'm doing wrong?

Bah my post got deleted. Thanks for the table. By the way, on page 244 Kaplan Premier they—incorrectly—state that, "...you can combine these rules above with factorization tom figure out whether a number is divisible by other numbers." the example given is that 8184 is divisible by 44 because it is divisible by (4 and 11). While this may be true in this case, it should advise that you use prime factorization.

For example, 36 is divisible by both 4 and 2, yet it is not divisible by 8. Rather, it should be divisible by 2 three times.

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