# What is the math behind this trick?

**URL:** <https://boards.straightdope.com/t/what-is-the-math-behind-this-trick/345792>\
**Category:** Factual Questions\
**Created:** [February 23, 2006, 3:27pm UTC](https://boards.straightdope.com/t/what-is-the-math-behind-this-trick/345792 "2006-02-23T15:27:55Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![stpauler](https://avatars.discourse-cdn.com/v4/letter/s/9d8465/32.png) [@stpauler](https://boards.straightdope.com/u/stpauler)\
**Post date:** [February 23, 2006, 3:27pm UTC](https://boards.straightdope.com/t/what-is-the-math-behind-this-trick/345792/1 "2006-02-23T15:27:55Z")

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There’s obviously a formula for[this.](http://digicc.com/fido/)

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**Author:** ![beagledave](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@beagledave](https://boards.straightdope.com/u/beagledave)\
**Post date:** [February 23, 2006, 3:38pm UTC](https://boards.straightdope.com/t/what-is-the-math-behind-this-trick/345792/2 "2006-02-23T15:38:17Z")

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> [@stpauler](#):
>
> There’s obviously a formula for[this.](http://digicc.com/fido/)

It’s a variation of the multiples of 9 trick

[http://www.snopes.com/humor/iftrue/psychic.htm](http://www.snopes.com/humor/iftrue/psychic.htm)

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**Author:** ![stpauler](https://avatars.discourse-cdn.com/v4/letter/s/9d8465/32.png) [@stpauler](https://boards.straightdope.com/u/stpauler)\
**Post date:** [February 23, 2006, 3:45pm UTC](https://boards.straightdope.com/t/what-is-the-math-behind-this-trick/345792/3 "2006-02-23T15:45:04Z")

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Thanks!