Tuesday, August 9, 2011

Divisibility Math Tricks

Miss Charlotte has started math for this year since this is her first year of working independently with the Saxon DIVE cd.

It is going very well!

One of the things we talked about today, after correcting her work, are the little tricks to determine if a large number is divisible by 3 or 7 or 9.

About (dot) com has a great review of the rules:

Dividing by 2

All even numbers are divisible by 2. E.g., all numbers ending in 0,2,4,6 or 8.

Dividing by 3

Add up all the digits in the number.
Find out what the sum is. If the sum is divisible by 3, so is the number
For example: 12123 (1+2+1+2+3=9) 9 is divisible by 3, therefore 12123 is too!

Dividing by 4

Are the last two digits in your number divisible by 4?
If so, the number is too!
For example: 358912 ends in 12 which is divisible by 4, thus so is 358912.

Dividing by 5

Numbers ending in a 5 or a 0 are always divisible by 5.

Dividing by 6

If the Number is divisible by 2 and 3 it is divisible by 6 also.

Dividing by 7

Take the last digit in a number.
Double and subtract the last digit in your number from the rest of the digits.
Repeat the process for larger numbers.
Example: 357 (Double the 7 to get 14. Subtract 14 from 35 to get 21 which is divisible by 7 and we can now say that 357 is divisible by 7.

(There is another test for 7 that is even more complicated. You can check out the original entry here.)

Dividing by 8

This one's not as easy, if the last 3 digits are divisible by 8, so is the entire number.
Example: 6008 - The last 3 digits are divisible by 8, therefore, so is 6008.

Dividing by 9

Almost the same rule and dividing by 3. Add up all the digits in the number.
Find out what the sum is. If the sum is divisible by 9, so is the number.
For example: 43785 (4+3+7+8+5=27) 27 is divisible by 9, therefore 43785 is too!

Dividing by 10

If the number ends in a 0, it is divisible by 10.

Happy calculating!

All information taken from About.com and are not my original thoughts or work.


MarshaMarshaMarsha said...

okay, now that is pretty cool!

Kathi said...

...my head hurts.

Gonna share with Kylie, thanks!

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