Brain Teasers
Friday the 13th
When's the next year when there won't be a Friday the 13th?
Answer
Never.The 13th of each month will fall on different days, according to a complex pattern which takes several years to repeat, because of Leap Years. However, each day of the week comes up as the 13th within the first 9 or 10 months every year.
Assign A, B, C, D, E, F, & G randomly to Sun, Mon ... Sat.
If we assume, in a non leap year, that:
* 13 Jan is Day A, then
* 13 Feb is Day D,
* 13 Mar is Day D,
* 13 Apr is Day G,
* 13 May is Day B,
* 13 Jun is Day E,
* 13 Jly is Day G,
* 13 Aug is Day C,
* 13 Sep is Day F
Thus all seven possible days of the week have been used, within a 9-month period.
Similarly, in a leap year:
* 13 Jan is Day A, then
* 13 Feb is Day D,
* 13 Mar is Day E,
* 13 Apr is Day A,
* 13 May is Day C,
* 13 Jun is Day F,
* 13 Jly is Day A,
* 13 Aug is Day D,
* 13 Sep is Day G,
* 13 Oct is Day B,
Thus all seven possible days of the week have been used, within a 10-month period.
Hide Answer Show Answer
What Next?
View a Similar Brain Teaser...
If you become a registered user you can vote on this brain teaser, keep track of which ones you have seen, and even make your own.
Solve a Puzzle
Comments hidden to avoid spoilers.
Follow Braingle!