I’ve seen puzzles of this type before but never remember how they’re constructed. Help!
Due to clever mathematics, the answer is always 9, 13, 18, 27, 36, 43, 45, 54, 63, 72 or 81. In the chart, these 12 numbers always are associated with ONE symbol (but the symbol changes everytime you access the page).
Actually, the only possibilities are multiples of nine. But it is a clever trick.
But more important, how did you come across this puzzle? I ask because just today, I got an email from my dad with the very same link. He rarely emails me, and when I saw the OP I was almost sure it the same. ** straykat23**, are you my daddy? But seriously, I’m interested in knowing how this puzzle was propagated around the 'net. Any comments?
Someone sent this to me about a week ago… …after a couple times I checked and no matter which pair of numbers I picked it was always the same symbol. I also notice the symbol changed each time I accessed the page.
Very simple. But at first it seemed impossible. 
Not 13!
In general, if you pick a two-digit number ab, then that number is really (10a+b). Then subtract (a+b), yielding 9a. So the number you end up with is always nine times the first digit of your original pick. Notice in the puzzle that all the multiples of 9 share the same symbol.
Some other discussion of this page
That doesn’t really work for me consistently. The first time I chose 49 - (4+9)=36 and it was doing something goofy then it started working. Then I chose 59 - (5+9) = 45 and it was still not working right. It worked fine when I chose numbers that didn’t result in a double digit number when adding the two digits together however.
Re: notcynical and olefin
Yes - I received 3 e-mails about the “mind reading trick” today (March 11) although the trick has bee around for a while. It’s amazing how quickly a webpage fad can quickly spread in 24 hours, and just as quickly fade. Only to be revived a few weeks or months later. I guess it’s based on the pyramid effect in e-mail , you send something interesting to everybody in your address book and by the end of the day, 20 million people are reading it. Kind of frightening, huh?
Same goes with e-mail hoaxes (the Outback Free Dinner), e-mailed blonde jokes and viruses.
Thanks for the reply, bradwalt. Oh, while I’m here, could you please fill me on the details of the “Outback Free Dinner”? Are you talking about the steak house? I’d love to get a free meal there!