Fwd: A practical tale of hacking with Revolution

[Geoff Canyon]

Sarah pointed out something I missed in solving the puzzle: that 0 and 
9 leave the same remainder when divided by 9, and are therefore 
indistinguishable. That's why the program has to ask that you not 
eliminate a 0. It could just as easily ask that you not eliminate a 9, 
but that would be leading you toward the solution.

Begin forwarded message:

> From: Sarah <sarahr at genesearch.com.au>
> Date: Mon Sep 22, 2003  9:33:21  PM America/Los_Angeles
> To: Geoff Canyon <gcanyon at inspiredlogic.com>
> Subject: Re: A practical tale of hacking with Revolution
>
> On Monday, September 22, 2003, at 12:04  pm, Geoff Canyon wrote:
>
>> So, my dad sent me this web site with the question: how does it work?
>>
>> http://digicc.com/fido/
>>
>> The site asks you to pick a number,
>> jumble the digits to make a second number,
>> subtract the smaller from the larger,
>> cross out a digit (not 0),
>> and enter the rest.
>> It then tells you which number you picked.
>>
>> For those of you who like to be mystified, or want to figure it out 
>> yourselves, read no further. Others, scroll down (there's transcript 
>> at the end).
>>
> Hi Geoff,
>
> I enjoyed this one. I had come across it before as my kids are into 
> all the "Horrible" books and this appeared in "Horrible Maths".
>
> I think the reason they don't allow you to pick 0 is that there is no 
> way to distinguish between 0 and 9. The way they did it in the book 
> (without your wonderfully detailed explanation) was to keep adding the 
> digits together until you were down to a single digit. Then if that 
> last digit was a 9, the answer was 9, otherwise, the answer was 9 - 
> the last digit.
>
> Cheers,
> Sarah
> sarahr at genesearch.com.au
> http://www.troz.net/Rev/

regards,

Geoff Canyon
gcanyon at inspiredlogic.com