Enigma 1047: Friday the 13th
27 August 2018
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From New Scientist #2203, 11th September 1999 [link]
Someone has told George that the 13th of a month is more likely to be a Friday than any other day of the week, but he is not sure whether he should believe it.
This year, 1999, the date Friday the 13th occurs only once, in August. Last year, 1998, it occurred three times, in February, March and November. It is clearly somewhat irregular.
If George choose a random month in a random year on the Gregorian calendar, what is the probability that it will include Friday the 13th? Please give your answer as a fraction in its lowest terms.