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Author Topic: For math geeks-how do they do this?
snupy
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Icon 1 posted June 19, 2006 12:25      Profile for snupy   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
http://digicc.com/fido/

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drunkennewfiemidget
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Icon 1 posted June 19, 2006 12:40      Profile for drunkennewfiemidget     Send New Private Message       Edit/Delete Post   Reply With Quote 
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Spiderman

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Icon 1 posted June 19, 2006 12:42      Profile for Spiderman     Send New Private Message       Edit/Delete Post   Reply With Quote 
Fascinating.

The sum of the digits of the subtracted number always equal 18. By providing the site with the other digits of the number, it's easy enough to deduce what the remaining digit is.

I'm not entirely sure just *why* those digits add up to 18 though. [Razz]

Clever, nonetheless. [Smile]

Edit: dnm: argh, you would have to just paste a link to the solution wouldn't you? [Razz] Referring to someone elses solution is just too boring. [Roll Eyes] [Geek]

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Math problems? Call 1-800-[(10x)(13i)^2]-[sin(xy)/2.362x]

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drunkennewfiemidget
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Icon 1 posted June 19, 2006 12:47      Profile for drunkennewfiemidget     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Spiderman:
Fascinating.

The sum of the digits of the subtracted number always equal 18. By providing the site with the other digits of the number, it's easy enough to deduce what the remaining digit is.

I'm not entirely sure just *why* those digits add up to 18 though. [Razz]

Clever, nonetheless. [Smile]

Close. The number is always the difference between the sum of the digits and next multiple of 9.
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Spiderman

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Icon 1 posted June 19, 2006 13:02      Profile for Spiderman     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by drunkennewfiemidget:
quote:
Originally posted by Spiderman:
Fascinating.

The sum of the digits of the subtracted number always equal 18. By providing the site with the other digits of the number, it's easy enough to deduce what the remaining digit is.

I'm not entirely sure just *why* those digits add up to 18 though. [Razz]

Clever, nonetheless. [Smile]

Close. The number is always the difference between the sum of the digits and next multiple of 9.
Yes, after playing with it a bit more, it quickly became evident that my arrival at the number 18 was correct, to a point, but not adequate to account for smaller numbers and numbers with small digits (or larger numbers for that matter).

It's a delicious oddity that the absolutely value of the difference between two permutations of a set of digits always equals a multiple of 9.

Oh, and now I see you've posted a solution. Lovely. [Razz]

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Math problems? Call 1-800-[(10x)(13i)^2]-[sin(xy)/2.362x]

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snupy
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Icon 14 posted June 21, 2006 16:50      Profile for snupy   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
fascinating...like a foreign language, but fascinating.

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Stormtalon
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Icon 1 posted June 23, 2006 14:32      Profile for Stormtalon   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
I think it has to do a lot with the fact (haven't found a formal proof yet -- then again, haven't looked either) that:

For every numbering system of base (n) utilizing numerals 0 thru (n-1), the digits in each multiple of the terminal digit (n-1) when added together always equal a lesser multiple of (n-1).

So, multiply 9 (decimal system) by anything. Add the digits. The result is a multiple of 9. In hexadecimal, the same holds true for multiples of E. Haven't worked up thru numbering systems beyond hex, yet, but I'm sure it continues to hold true.

If someone can find a formal proof, that'd rock. Otherwise, I'll probably e-mail Polymath or Good Math, Bad Math blogs to see what those guys say. [Smile]

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