Divisibility Tests for 7 (1 Viewer)

[GaDaMIt]

I'm a 3u preliminary student..

Today we were doing some basic stuff, the divisibilty tests eg for 9 - the sum of the digits must be divisible by 9.

For 7 however, our teacher told us to look it up ourselves because it is "all very complicated and some fantastic formulas have been worked up".

So naturally I came here to get a divisibility method of 7.

Thanks for your help

 

[Raginsheep]

Search google for "Divisibility Test 7". The answers will be there.

 

[black_kat_meow]

To find out if a number is divisible by seven, take the last digit, double it, and subtract it from the rest of the number.
Example: If you had 203, you would double the last digit to get 6, and subtract that from 20 to get 14. If you get an answer divisible by 7 (including zero), the original number is divisible by seven. If you don't know the new number's divisibility, you can apply the rule again.

Hope that helps

 

[darkliight]

From http://home.egge.net/~savory/maths1.htm

Double the last digit and subtract it from the remaining leading truncated number. If the result is divisible by 7, then so was the original number. Apply this rule over and over again as necessary.

Example: 826.
Twice 6 is 12. So take 12 from the truncated 82. Now 82-12=70. This is divisible by 7, so 826 is divisible by 7 also.

 

[black_kat_meow]

That was effectively what I just said. Just with less fancy words!

 

[darkliight]

Yeah I was just slow, then figurered I might as well leave the link there