# Answer to Parade Magazine "Ask Marilyn" puzzler.

**URL:** <https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762>\
**Category:** Factual Questions\
**Created:** [October 20, 2008, 5:41pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762 "2008-10-20T17:41:03Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![randwill](https://avatars.discourse-cdn.com/v4/letter/r/a3d4f5/32.png) [@randwill](https://boards.straightdope.com/u/randwill)\
**Post date:** [October 20, 2008, 5:41pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/1 "2008-10-20T17:41:03Z")

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Help a dumb guy out here. In Sunday’s “Ask Marilyn” column in Parade Magazine I read;

Here’s a puzzler: What does this set of equations describe?  
1 + 2 = 3  
3 + 4 = 7  
5 + 6 = 11  
7 + 8 = 3  
9 + 10 = 7  
11 + 11 = 10  
Answer: The arithmetic of an ordinary 12-hour clock.

I don’t get it.

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**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\
**Post date:** [October 20, 2008, 5:42pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/2 "2008-10-20T17:42:47Z")

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8 hours after 7 PM, it’s 3 AM, right?

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**Author:** ![Captain\_Amazing](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@Captain\_Amazing](https://boards.straightdope.com/u/Captain_Amazing)\
**Post date:** [October 20, 2008, 5:42pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/3 "2008-10-20T17:42:43Z")

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if it’s 1, then 2 hours from now it’ll be 3.  
If it’s 3, 4 hours from now it’ll be 7.  
If it’s 5, 6 hours from now it’ll be 11.  
If it’s 7, 8 hours from now it’ll be 3.  
If it’s 9, 10 hours from now it’ll be 7  
If it’s 11, 11 hours from now it’ll be 10.

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**Author:** ![Ace309](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ace309/32/1050_2.png) [@Ace309](https://boards.straightdope.com/u/Ace309)\
**Post date:** [October 20, 2008, 5:43pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/4 "2008-10-20T17:43:31Z")

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7 am + 8 hours = 1500, or 3 PM

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**Author:** ![Antigen](https://avatars.discourse-cdn.com/v4/letter/a/848f3c/32.png) [@Antigen](https://boards.straightdope.com/u/Antigen)\
**Post date:** [October 20, 2008, 5:44pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/5 "2008-10-20T17:44:02Z")

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If it’s 1:00, add 2 hours, get 3:00.

And so on… once you hit the bigger numbers, it’s 7:00, add 8 hours, it’s 3:00.

ETA: I see that I am still very slow with non-digital clocks…

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**Author:** ![pandNH4](https://avatars.discourse-cdn.com/v4/letter/p/eb8c5e/32.png) [@pandNH4](https://boards.straightdope.com/u/pandNH4)\
**Post date:** [October 20, 2008, 5:47pm UTC](https://boards.straightdope.com/t/answer-to-parade-magazine-ask-marilyn-puzzler/468762/6 "2008-10-20T17:47:59Z")

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This is called modular arithmetic, actually a very important part of math  
([Modular arithmetic - Wikipedia](http://en.wikipedia.org/wiki/Modular_arithmetic))