# from the "weird" department

Discussion in 'Community' started by dragula53, May 2, 2004.

1. ### dragula53 macrumors regular

Joined:
Jun 23, 2003
#1
http://digicc.com/fido/

creepy

computers that know what I am thinking scare me.

actually, it's just neat.

2. ### Juventuz macrumors 6502a

Joined:
Dec 4, 2002
Location:
Binghamton
#2
It got it right only 2 out of 5 times for me.

### Staff Member

Joined:
Sep 19, 2002
Location:
Los Angeles
#3
I didn't work out the math (no time left, Alias is about to start), but I think when you subtract a scrambled number from the same digits in another order, the result is a multiple of 9, so the digits add up to 9. Therefore, if you type all the digits but one you circled, the other one is 9 minus those. Since they might add up to more than 9, I've probably oversimplified.

4. ### rainman::|:| macrumors 603

Joined:
Feb 2, 2002
Location:
iowa
#4
i think you're right except that the numbers add up to 18, tho i don't know why (years ago we studied the 9 effect but i don't remember any of the specifics). Maybe it's 9 when it's two digits, 18 when it's four digits. That kind of makes sense.

paul

Joined:
Jan 5, 2004
6. ### Daveman Deluxe macrumors 68000

Joined:
Jun 17, 2003
Location:
Corvallis, Oregon
#6
If the sum of the digits in any number adds up to three or a multiple of three, the number is a multiple of three. If the sum of the digits in any number adds up to nine or a multiple of nine, the number is a multiple of three.

If the number is even, a multiple of two.

If the tens and ones places are divisible by four, a multiple of four.

If the number is even and divisible by three, it is divisible by six.

There's some crazy trick for seven. It was really cool but I forget what it is.

7. ### Abstract macrumors Penryn

Joined:
Dec 27, 2002
Location:
Location Location Location
8. ### Powerbook G5 macrumors 68040

Joined:
Jun 23, 2003
Location:
St Augustine, FL
#8
Maybe I did the math wrong, but it didn't guess the correct number for me.

9. ### MongoTheGeek macrumors 68040

Joined:
Sep 13, 2003
Location:
Its not so much where you are as when you are.
#9
Its an old accountant trick. if you are off by a multiple of 9 then you swapped digits someplace.

It works because the error (for a two digit number extrapolating to additional digits is left as an exercise to the reader) is as follows.

For two digits M,N where m>n the larger number would be

10M+N

the smaller Number would be

10N+M

the error is

9M-9N or 9(M-N) which is always an integer multiple of 9.

Since you can transpose any two adjacent digits a series of swaps will get the digits into place leaving you off by various multiples of 9.

10. ### MongoTheGeek macrumors 68040

Joined:
Sep 13, 2003
Location:
Its not so much where you are as when you are.
#10

The only real failure is the confusion between 0 and 9 which is why it doesn't allow you to pick a 0

11. ### crenz macrumors 6502a

Joined:
Jul 3, 2003
Location:
Shanghai, China
#11
Wow, that's nice! There are a couple ones like that, but this is one of the more elaborate ones.

### Staff Member

Joined:
Sep 19, 2002
Location:
Los Angeles
#12
There was a clever web site that had a "guess which card in the deck you chose" trick, using only face cards (jacks, queens, kings). I've lost track of the URL, but it was equally fun to figure out how it worked.

13. ### virividox macrumors 601

Joined:
Aug 19, 2003
Location:
Manila - Nottingham - Philadelphia - Santa Barbar
#13
i love trying to figure these things out. sometimes i get really stumped tho.