# How does this work?

**URL:** <https://boards.straightdope.com/t/how-does-this-work/157497>\
**Category:** Factual Questions\
**Created:** [February 25, 2003, 10:32pm UTC](https://boards.straightdope.com/t/how-does-this-work/157497 "2003-02-25T22:32:08Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Alma\_Tender\_Love](https://avatars.discourse-cdn.com/v4/letter/a/eb9ed0/32.png) [@Alma\_Tender\_Love](https://boards.straightdope.com/u/Alma_Tender_Love)\
**Post date:** [February 25, 2003, 10:32pm UTC](https://boards.straightdope.com/t/how-does-this-work/157497/1 "2003-02-25T22:32:08Z")

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Hi  
I’m new to these boards. Can someone please explain how [Flash Mind Reader](http://mr-31238.mr.valuehost.co.uk/assets/Flash/psychic.swf) works?  
Thanks in advance  
Alma xx

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**Author:** ![suranyi](https://avatars.discourse-cdn.com/v4/letter/s/e36b37/32.png) [@suranyi](https://boards.straightdope.com/u/suranyi)\
**Post date:** [February 25, 2003, 10:55pm UTC](https://boards.straightdope.com/t/how-does-this-work/157497/2 "2003-02-25T22:55:29Z")

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> [@](#):
>
> \*Originally posted by Alma Tender Love \*  
> \*\*Hi  
> I’m new to these boards. Can someone please explain how [Flash Mind Reader](http://mr-31238.mr.valuehost.co.uk/assets/Flash/psychic.swf) works?  
> Thanks in advance  
> Alma xx \*\*

It works because the number you end up with is always a multiple of 9, and in the chart all the multiples of 9 (9, 18, 27, etc.) have the same symbol.

Now why do you always end up with a multiple of 9? It has to do with a well-known property of numbers: If you add up all the digits in a number, and add up all the digits in the sum, continuing until you are left with one digit, then that one digit is the remainder of the original number when you divide by 9. (If the one remaining digit IS 9, then the orginal number was divisible by 9.)

So if you subtract that number from the original, you are subtracting the remainder from the original, which must leave a number divisible by 9.

Ed

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**Author:** ![Jack\_Batty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jack_batty/32/3201_2.png) [@Jack\_Batty](https://boards.straightdope.com/u/Jack_Batty)\
**Post date:** [March 8, 2003, 3:12pm UTC](https://boards.straightdope.com/t/how-does-this-work/157497/3 "2003-03-08T15:12:08Z")

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Here’s how the multiple of nine result was explained to me, as basically an algebra equation:

(I hope **typo mna** doesn’t mind me quoting him from another board. Eh, what do I care? What’s he going to sue me for? $12.50 in returnable bottles?)

“The instructions are to pick a number, add the digits and subtract from the original number. If _a_ is the tens place and _b_ is the ones place, the number is (_a_ times 10) + _b_. When you add the digits together, you get _a_ + _b_. Subtracting that from the original number gives the equation (_a_ times 10) + _b_ - (_a_ + _b_), which equals (_a_ times 9) + _b_ - _b_, or _a_ times 9. Which, as was pointed out, is a multiple of nine, so it always matches the symbols next to the multiples of nine.”