# Re: OT: Unbelievable

From: | Henrik Theiling <theiling@...> |

Date: | Friday, June 6, 2003, 20:03 |

Hi!
James Worlton <jamesworlton@...> writes:
> doing what the site says will always result in a
> multiple of 9. I can't prove it mathematically, but a
> few trials showed this to be the case:
> 15 -> 1+5 = 6; 15-6=9
> 99 -> 9+9 = 18; 99-18=81
> 47 -> 4+7 = 11; 47-11=36
> I think it is called the rule of 9s or something like
> that.

Easy proof, simply calculate:
For any number 'xy', i.e. x*10+y, x,y\in{0,..,9}:
x*10+y - (x+y) = x*10 - x - y = x*10 - x = x*9.
Since x is integer, x*9 is divisible by 9.
And you can even predict that the result is x*9, not only
divisible by 9.
<http://mr-31238.mr.valuehost.co.uk/assets/Flash/psychic.swf>
I could not see that because of Flash. I hope I calculated the right
thing... :-)))
**Henrik

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