Math Magic: Mind-Reading Web Sites

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crystal.jpgThere are several sites online that claim to read your mind. A typical scenario is as follows:

You pick a two-digit number, add the digits, and subtract that sum from your original number. You then look up this difference in a chart of numbers [from 0 to 99] and make note of the associated symbol. The site will then tell you what symbol you noted.

You can play this game for yourself at (for example) MysticalBall.com or The Ghost Whisperer’s Crystal Ball on CBS’s website.

Magic on the Internet? A mathematical trick? See if you can figure it out (or post a comment if you need a hint!).

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3 Responses to “Math Magic: Mind-Reading Web Sites”

  1. Ξ Says:

    If you want a hint, look at the first comment in the Mind-Reading post here.

  2. blah blah blah Says:

    ive dun it b4 but i stil dont get it………….

  3. Ξ Says:

    It turns out that if you take a number, add the digits, and then subtract, the difference that you get is ALWAYS a multiple of 9. Looking at 34, the sum of the digits is 3+4=7 and 34-7=28, which is a multiple of 9. Similarly, the digits of 56 are 5+6=11 and 56-11=45, which is a multiple of 9.

    If you look at any of those mind-reading sites, even though there are a bunch of symbols, the multiples of 9 [9, 18, 27, 36, etc.] all have the same symbol, and that’s the one that the computer picks. (Usually this symbol changes each time you play the game, just to be sneaky.)

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