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More ways to get Braingle...

Friday the 13th



 

Puzzle ID:#49250
Fun:*** (2.55)
Difficulty:** (1.41)
Category:Other
Submitted By:babyjuice*

 

 

 



When's the next year when there won't be a Friday the 13th?




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Comments

randomgirl*no*
Jan 15, 2012

I did not get that!
babyjuice*
Jan 15, 2012

Really, thats the best you could think of to say, I thought it was OK for a first teaser.

Attention everyone else, she is my friend and is really just trying to annoy me. Please post if you like or hate it
randomgirl*no*
Jan 15, 2012

This is kind of cool!
zanner2001
Jan 15, 2012

u 2 r crazy i like this one its ok
dalfamnestAnz*
Jan 15, 2012

A good teaser, but it requires a lot more thought than you suggest! The answer is right, but your use of probability is not correct - I have submitted a suggestion for a better explanation.

Keep up the good work!
Mathgeek007tca*
Jan 16, 2012

Assume it's not a leap year.

When a month starts on Sunday, it will have a Friday the 13th.

Monday, Thursday, Thursday, Sunday, Tuesday, Saturday, Monday, Tursday, Saturday, Tuesday, Thursday, Sunday.

As you see, no Wednesday.

2 days before Friday.

2 days before Monday, the day I started with. There will be no Friday the 13th.

The next non-leap year that starts on Saturday will not have a Friday the 13th.
jhosekus*
Jan 17, 2012

@Mathgeek007 - In a leap year that starts on a Saturday, October will have a Friday the 13th
jhosekus*
Jan 17, 2012

Oops. Sorry i mis-read your comment "Non-leap year"
jhosekus*
Jan 17, 2012

in a non-leap year that starts on a Saturday, May will have a Friday the 13th
jhosekus*
Jan 17, 2012

When a non-leap year starts on a Monday the first day of each month (Jan-Dec) fall on
Mon,Thurs,, Thurs, Sun, Tues, Fri,
Sun, Wed, Sat, Mon, Thur, Sat

In a leap year starting on a Monday the 1st day of the months fall on
Mon, Thur, Thur, Mon, Wed, Sat
Mon, Thur, Sun, Tues, Fri, Sun

In both cases there is a least 1 month starting on each day of the week. So, if the starting day of the year is shifted there will still be at least one month starting on a Sunday meaning there will be at least one Friday the 13th in any year.
babyjuice*
Jan 17, 2012

You beat me to post! Also, the year is 2022
wolfpackw2*us*
Feb 02, 2012

There are only 14 possible distinctive calendars. One for each day Jan. 1 can start on for a non-leap year (7), and another set for leap year (7). If you look at each of these 14 possibilities, there is ALWAYS a year that has at least 1 month that will start on Sunday (resulting in Friday the 13th). Therefore, there will ALWAYS be a Friday the 13th in every year. It has nothing to do with probability.
dalfamnestAnz*
Feb 03, 2012

Sorry, guys, every year MUST have a Friday 13th. Mathgeek - you need to check your logic, and others do, too. May 13, 2022 will be a Friday.

The answer is correct, but the reasoning is totally wrong, as this has nothing to do with probability. The theory of probability relies on events being random ... and this is not random.
RGW4Aus*
Apr 05, 2012

I got it right only because there has always been to be at least one Friday the 13th in my 56+ years of living. Some years more than others. Nice work!



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