# Mathematics

**URL:** <https://boards.straightdope.com/t/mathematics/619411>\
**Category:** Factual Questions\
**Created:** [April 20, 2012, 2:19pm UTC](https://boards.straightdope.com/t/mathematics/619411 "2012-04-20T14:19:13Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\
**Post date:** [April 22, 2012, 8:38pm UTC](https://boards.straightdope.com/t/mathematics/619411/21 "2012-04-22T20:38:10Z")

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> [@Indistinguishable](#):
>
> Eh, for aesthetic symmetry, I should have written that as (10x + y)/7 = 3(x - 2y)/7 + x + y and (x - 2y)/7 = -2(10x + y)/7 + 3x.
> 
> (Also, sorry I stole your thunder, **Andy L**.)

No problem

[QUOTE=Chronos]  
I would phrase the reason as “you’re subtracting off a multiple of 21, and then dividing by ten”.  
[/QUOTE]

That’s how I would have explained it.

You can construct similar tests for divisibility by 13 and 17, by the way.

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**Author:** ![glee](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@glee](https://boards.straightdope.com/u/glee)\
**Post date:** [April 23, 2012, 10:41pm UTC](https://boards.straightdope.com/t/mathematics/619411/22 "2012-04-23T22:41:20Z")

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> [@Exapno\_Mapcase](#):
>
> QG may mean:  
> Quantum gravity, a physics theory unifying relativity (gravity) and quantum mechanics  
> Queen’s Gambit, a chess opening move  
> …

In over 50 years of playing, teaching and reading about chess, I have **never** seen QG used as an abbreviation for Queen’s Gambit.

(In chess openings classification, it would be [ECO D2 + D3](http://www.chess-theory.com/enva0299y0_links_chess_theory_eco_codes_base.php).)

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**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\
**Post date:** [April 23, 2012, 11:26pm UTC](https://boards.straightdope.com/t/mathematics/619411/23 "2012-04-23T23:26:52Z")

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> [@glee](#):
>
> In over 50 years of playing, teaching and reading about chess, I have **never** seen QG used as an abbreviation for Queen’s Gambit.

NIC uses QG for the QGA, and QO for the QGD.

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