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       Click on the above link, observe it carefully and then try to find out the secret behind this mind reading puzzle! Too lazy to try? no problem buddy, here is also the explanation for you...but I hope, you'll visit the puzzle site before going through this explanation and it will be more helpful for your better understanding... 

     Let me suppose the two digit number is :  xy... that's the first digit is : x and the second digit is : y. So its numerical value is : 10x+y...and the sum of the two  digits is : x + y.

     Hence, the difference is :   (10x + y)  - (x + y)
                                             =  10x + y  - x - y 
                                             =  9x

    So whatever my two digit number is, the  difference is always divisible by 9.

    For example, if your two digit number is: 49... then, in that case, x = 4 and y = 9. So, it's numerical value is : 10 * 4 + 9  and the sum of the two  digits is : 4 + 9 = 13. Here, the difference is : 49 - 13 = 36, which is divisible by 9  (9 * 4 = 36).

      So whatever your two digit number is, the  difference will always be any of 9, 18, 27, 36, 45, 54, 63, 72 and 81... but neither 90 nor 99... since, x is a single digit number, 9x can never be 90 (9*10) or 99 (9*11). Hence, there are only nine possible numbers to be put on your mind. So while there are lots of symbols provided, you only have to worry about nine of them: 9, 18, 27, 36, 45, 54, 63, 72 and 81.

     And now, if you look close, you'll find: despite the fact that the symbols are randomized, there is a peculiar consistency in the Mind Reader: multiples of 9 (except for 90 and 99) always share the same symbol. You can refresh the page again n again...but you'll see a new symbol shared by all these nine numbers. And this is the trick behind this Mind Reading puzzle....