Great math trick

[Shawn Preston]

Hello fellow forumers,
I came across this math trick. Check it out... I'm totally fooled.

Shawn


http://digicc.com/fido/

[Bill Mullins]

Any time you take a number this size, scramble the digits, and subtract it from the unscrambled, you get a number which is an integral multiple of nine.

Any number which is a multiple of nine, if you add the digits together, you get another (smaller) multiple of nine. You can recursively do this until you get nine (called "casting out nines").

The 7up cartoon guy takes the numbers you enter, casts out nines until he has a number less than 9, and subtracts it from nine. The answer is the digit you circled.

[Guest]

Thanks for the exposure!

[Guest]

yea, how der he...
:sleep:

[Ryan Matney]

I think that's one of the best online tricks I have seen.

As a historical aside (and because I'm reading about it just as this came up) Lewis Carroll used to perform this very trick for his mathematics class at Christ Church, Oxford in 1856!

Carroll also performed a sort of rapid addition stunt in which the spectator and magician each alternate writing 5 digit numbers. The magician instantly totals all correctly. This also uses the unusual properties of 9.

[Mike Powers]

Just as a mathematical aside - The reason that 9 is cool is that we have a base 10 system. If we had a base 16 system, like computers folks use, the magic number would be 15. It's always 1 less than base of the number system. Ultimately the reason for these properties is that 10 MOD 9 = 1 and thus 10, 100, 1000 etc all = 1 mod 9 and thus any number is "equivalent" to the sum of its digits. Example: 3517 = 3x1000 + 5x100 + 1X10 + 7. But since 1000=10^3 and 100=10^2 etc. and 10 mod 9 = 1, 1000 = 1 etc....

[Jim Maloney_dup1]

Originally posted by Mike Powers:
If we had a base 16 system, like computers folks use, the magic number would be 15.

Or...you know...'F'.

Base 16 = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

-Jim, Computer Nerd Extraordinaire

There are only 10 types of people in the world - Those who understand binary, and those who don't.

[Guest]

Originally posted by Jim Maloney:

Originally posted by Mike Powers:
[b]If we had a base 16 system, like computers folks use, the magic number would be 15.

Or...you know...'F'.

Base 16 = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

-Jim, Computer Nerd Extraordinaire

There are only 10 types of people in the world - Those who understand binary, and those who don't. [/b]

Jim,

Next time you see a group of people marching and calling cadence, move alongside and call cadence in binary!

Oh-oh, oh-one, one-oh, one-one!

Drives em nuts!

Or, you could count to four, in binary, on your fingers as an insult.

Zero - no fingers up
One - extend thumb
two - retract thumb, extrnd index finger
three - extend both index finger and thumb
four, retract thumb & forefinger, extend middle finger. :eek:

Computer geeks usually love that one, but, as the folks as Iomega say: "Disk, too, shall pass!" ;)

Lee Darrow, C.Ht.
http://www.leedarrow.com

[Kamus]

if the magic number is always one less than the system base, does that work in binary, where the magic number would be zero?

[CHRIS]

binary has a system base of 2 and magic number would be 1.

Chris
preserving magic one book at a time

[Kamus]

Well Duh! What in $%^& was I thinking?- but of course! Thanks for setting me straight :p