# Magic Trick: how?

**URL:** <https://boards.straightdope.com/t/magic-trick-how/296748>\
**Category:** Factual Questions\
**Created:** [March 29, 2005, 8:51am UTC](https://boards.straightdope.com/t/magic-trick-how/296748 "2005-03-29T08:51:19Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Boppy](https://avatars.discourse-cdn.com/v4/letter/b/df788c/32.png) [@Boppy](https://boards.straightdope.com/u/Boppy)\
**Post date:** [March 29, 2005, 8:51am UTC](https://boards.straightdope.com/t/magic-trick-how/296748/1 "2005-03-29T08:51:19Z")

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This link was sent to me recently and I fell for it. How does it work?

[http://www.magictricks.co.uk/mindreader/MagicTricksMR](http://www.magictricks.co.uk/mindreader/MagicTricksMR)

Give it a moment to load …

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**Author:** ![Ponster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponster/32/17081_2.png) [@Ponster](https://boards.straightdope.com/u/Ponster)\
**Post date:** [March 29, 2005, 9:01am UTC](https://boards.straightdope.com/t/magic-trick-how/296748/2 "2005-03-29T09:01:58Z")

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[Answer](http://chuck.charleshart.net/Mathomagic.php)

> [@](#):
>
> 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.

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**Author:** ![Fromage\_A\_Trois](https://avatars.discourse-cdn.com/v4/letter/f/a88e57/32.png) [@Fromage\_A\_Trois](https://boards.straightdope.com/u/Fromage_A_Trois)\
**Post date:** [March 29, 2005, 9:05am UTC](https://boards.straightdope.com/t/magic-trick-how/296748/3 "2005-03-29T09:05:52Z")

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There’s only a limited number of answers - any number in the twenties will return ‘18’, anything in the thirties ‘27’ etc. You’ll notice there’s also only a limited number of symbols and - surprise! - all the numbers that are a valid solution have the same symbol (although it changes each time you try).

Drat, on preview, I see Ponster got there first.

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**Author:** ![Boppy](https://avatars.discourse-cdn.com/v4/letter/b/df788c/32.png) [@Boppy](https://boards.straightdope.com/u/Boppy)\
**Post date:** [March 29, 2005, 9:21am UTC](https://boards.straightdope.com/t/magic-trick-how/296748/4 "2005-03-29T09:21:15Z")

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Thanks, Bras and Ponster. Now I can sleep easy.  
Cheers,  
B

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**Author:** ![Ponster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponster/32/17081_2.png) [@Ponster](https://boards.straightdope.com/u/Ponster)\
**Post date:** [March 29, 2005, 10:01am UTC](https://boards.straightdope.com/t/magic-trick-how/296748/5 "2005-03-29T10:01:40Z")

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The sneaky bit is that the image with the symbols changes each time that you play !