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 <[EMAIL PROTECTED]>
Date: Mon Sep 22, 2003  9:33:21  PM America/Los_Angeles
To: Geoff Canyon <[EMAIL PROTECTED]>
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
[EMAIL PROTECTED]
http://www.troz.net/Rev/
regards,

Geoff Canyon
[EMAIL PROTECTED]
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