Problem 4. The month of May in one year had four Mondays and four Fridays. What day of the week was May 1st?
Solution. May has 31 days. If the month has four Mondays and four Fridays, then there are also four Saturdays and Sundays. The days that fall on the 29th, 30th, and 31st occur five times in the month, so they are Tuesday, Wednesday, and Thursday. Therefore, May 29th is a Tuesday, and May 1st is also a Tuesday.