I'm pretty good with math, but I'll be damned if I can come up with the correct formula for how they figure this out. Any mathematicians out there? http://digicc.com/fido/ The damn thing got me three for three, so there must be some solid number manipulation going on behind it.

A friend of mine has solved it. Here's his solution: The difference between two four digit numbers that have the same digits results in a number that is a multiple of 9 every time. Since he tells you not to pick 0, that leaves only 9 digits to choose from, and only one will fit that solution.

pretty clever, but it wouldnt be tough to figure out, if you had a lot of time. when you have to type in your number, just start out with 1 and increase, and see what patterns emerge. its not magic, so there would definitely be patterns that you could determine what kind of formula was used. like every time the last number ends in 36, the result is an 8 or something like that. CJ

I had someone come to me with this last night, but the website makes it much clearer. I pondered on this some amount last night and it appears to be an interesting bit of coincidental digit relationships. However, I'm fairly certain that this always works as long as you don't choose a number with all the same digits (like 444) and that you don't remove a zero from the 'guess' portion of the call. From what we came up with last night, the subtracted value is always a multiple of 9. 614 as a base number will return 27 (9*3),153 (9*17), 198 (9*22), 450 (9*50), 468 (9*52). And a basic property of mulitples nine is that all of their digits summed are a product of nine. The part I couldn't figure is that they were always able to get a subtracted number that is a multiple of nine. However a brilliant coworker figured it out. Any number can be represented algebraically as a variable and the place, such as 1000a+100b+10c+d. Rearanging them would give you 1000c+100d+10a+b. Subtracting the two gives you 990a+99b-990c-99d. As you can see, this always gives you a resulting number with a multiple of nine 9*(110a+11b-110c-11d), which sets up the second part of the trick which plays on the property of nine. Neat trick, but I finally understand it.

That's cool stuff. I love things like this. I guess that's why I'm a die-hard computer geek/administrator. After all, computers are nothing but numbers. Thanks!

Ah the magic of Nine. Wasn't there a song about this on Saturday Morning cartoons way back. Kinda like a schoolhouse rock jingle thing. It was either 9 or 3. Same ting either way, 3 being a factor of nine and all.

OK, here is what I got. I can't explain it but it works. Follow the same steps as above. When you get to the step where a number has been circled; add up the remaining numbers. Then go to the first multiple of nine that is greater than that number and subtract. Voila you have your answer. For example, if the remaining digits are 4-6-0 then adding them together gives you 4+6+0=10. The first multiple of nine that is greater than 10 is 18. Thus 18-10=8. That's your answer 8! Works every time. Try it, enjoy it, be the hit at all the parties!! Ray :wink: