I didn't even do the maths, just clicked next and it worked..... weird....

http://www.milaadesign.com/wizardy.html

Spoooooky

Have seen a few versions of this

A much better internet trick (in my humble opinion) is built by Andy Naughton, who gave me permission to use it in this lecture. (Ref: http://www.cyberglass.co.uk/moviesflsh/mindreader.php),

Choose any two-digit number, add together both digits and then subtract the total from your original number. When you have the final number, find it on the chart and find the adjacent symbol (to the right). Concentrate on the symbol - concentrate - I'll read your mind. Your mind is like an open book - no, you're thinking of an open book - that's it! The symbol that you are thinking of is the "open book"!

If you haven't figured it out: within each group of ten numbers (the teens, twenties, etc.), subtracting the sum of the two digits produces the same number: a multiple of 9. All the teens map into 9, all the twenties map into 18, all the thirties map into 27, etc. All these numbers are the same symbol; all the other symbols are scattered randomly to confuse the pattern.

This example is especially good on the internet, because the pattern can be re-arranged between trials. A different symbol each time is then used for each iteration.

http://chuck.charleshart.net/Mathomagic.php

if you look at the last page, there are different numbers with the same symbol
you get different symbols each time because they change (e.g look at number 18, do it again and take another look at 18)