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jmc

number theory intermediate

Problem

In any month with a Friday the Thirteenth, what day of the week is the first day of the same month?
Solution
If the 13th is a Friday, then the 14th is a Saturday, and the 15th is a Sunday. Subtracting 14 days (which is two weeks), we find that the first of the month is also a .
Final answer
\text{Sunday}