Here's a nice stunt (and a very attractive implementation) to try out: digicc.com/fido/

... Doug

An oldie, but I like the out of "don't draw a circle around the zero, 'cause it's already a circle." Splendid nonsense.

Here's a less often seen version of the above:

Pick a 3 digit number. Form a new number by scrambling the digits. Now form yet a third number by scrambling the digits again. If you like, you can decide to scramble the digits so much that they return to their original positions. Now add the three numbers you've formed. Square the result. Now eliminate one of the digits . . .

Jack Shalom

I think most accountants are familiar with that "trick"...
once, all the serial numbers of bills in Israel could be used for that.
think about borrowing a bill - having the spec circle one digit (IN HIS MIND) and read out all the rest, and you instantly reveal the thought of digit...
as far as I know it is no longer like that for 5 or 6 years now. (... Ah the good old days.)

A nice version is if you have a number with the properties we want (algebraic sum to you know what), then that number can be multiplied by any number at all, totally unknown to you, and the result will still have the same algebraic sum.

I used to go round the table asking people for digits until the sum was right, or add a number when it was my turn. If people thought that was a bit cheeky, I would then say, "ok, think of any number in the world (not zero) and multiply the group selection by that, secretly on this calculator (or mobile phone)". You can then go straight into the business.

Hi Loz,

Can you explain this a little more?



Thanks in advance.

I once did this by having someone choose 5 different numbers between 2 and 15. The trick won't work every time, but you won't believe how often it will work.

... And, hey, if it was just a magic trick, it would work every time, wouldn't it?

And Tomas will probably tell us exactly how often it will work



Jack Shalom

It works about 11% of the time, but it made me experiment with trying to find other ranges of numbers and other amount of numbers they chose, that'd increase the probability of the sum being dividable by 9...but so far I've found nothing.

/Tomas

Tomas, it worked a lot more than 11% of the time. It worked a majority of the time!

They don't only need to pick 9. Any two multiples of 3 will work, as well! As long as they choose 9, or any two of the following...

3, 6, 12, 15

...then the trick wil work! And, with 5 different numbers needed in that small a range, it's hard to avoid either a 9, or 2 multiples of 3.

Ah, got it, Scott. I randomly took 5 numbers in the range and _added_ them together. I now understand that you take their product, which works a whooping 61% of the time if you take the numbers at random within the range. If you make the range 3 to 15 (inclusive) you'll get it 74% of the time. Beautiful idea, Scott.

/Tomas

PS Just simulated with 5 different two-digit numbers and got the rate to 66%

Hi Lamberto,

If you have a number whose algebraic sum is 9, then if you multiply that number by any other number, the result will still have an algebraic sum of 9. Worried, is this math or exposure?

If ya want to know what I mean by algebraic sum I'm sure it can be found somewhere.

Here is a simple proof for a two double-digit numbers to show you how the 7 up trick works.
Let the two digit number be N or x, y and the scrambled number N' or y, x.
N= 10x+y
N'=10y+x
N-N'=(10x+y)-(10y+x)= 10x+y-10y-x=9x-9y=9(x-y)
So the subtracted result will always be a multiple of 9. It is a known fact that the product you get when multiplying by 9 will have the sum of its digits add up to 9. This is the basis of casting out nines to check multiplication. Knowing the sum is nine will allow you to figure out the other digit. With a number having more digits the principle is the same but the trick more impressive.

Thanks Loz for your reply.

Nah, no one reads this math stuff anyway. You could reveal Copperfield's latest illusion here and it would pass by unnoticed. Just be sure to label it something like "Eighty Two Reasons Why I Love Trigonometry"

Jack Shalom

Thanks Count for pointing out the algebra... Interesting stuff.

I know this topic was started last October and hasn't seen any action in a while. However, I did this number effect over the Internet with a friend through Instant Messenger. It was great!! She was amazed that I knew what number she had circled!!!

Does anybody know of any websites where these kind of mathematical puzzles or effects can be found?? This is great stuff! Is there an effect using, for example, the Periodic Table of the Elements or something like that??

BTW, Apocalypse had a great way to make sure the number was both large and worked for this stunt.

Hand someone a calculator, and point out that the numbers on the calculators are in a 3 by 3 design:

789
456
123

Have them choose any row, column or diagonal of numbers (741 or 456 or 159, for example), and enter those three number in any order (If you chose the 159 diagonal, you can put in 159, 951, 591, 519, 195 or 951). Next, hit the multiplication key, and enter any other row, column or diagonal in any order.

Since you're multiplying two three digit numbers, you get a nice large, impressive number to perform the effect with, and the nature of the calculator layout means that the number will work for this effect.

<<Does anybody know of any websites where these kind of mathematical puzzles or effects can be found?? >>


Ummm . . . here.


Jack

The best classics can be found here:
http://rec-puzzles.org/
most with brief and correct solutions.

Nir

Almost forgot, another great place to spend many hours on is this one here:
http://www.cut-the-knot.org/content.shtml

enjoy

Thanks, Nir Dahan! That's the kind of reply I was looking for!