It is a certainty. There will always be at least one month that will meet the requirement.
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First we don't consider leap year There is 1 month with 28 days: february There are 4 months with 30 days: april, june, september, november So the probability to select a month with 28 or 30 days is (1+4)/12 = 5/12 If the year is a leap year then the probability is 4/12
There are 12 months to choose from There are 7 months with 31 days in them. The probability of choosing a 31-day month is 7/12.
Absolute certainty
The simplistic answer is that August will have five Mondays if the month starts on a Saturday, Sunday or Monday. Three days out of the seven so the probability is 3/7 = 0.4286. However, that assumes that any day of the week is equally likely and that is not the case. The impact of leap years and their 2800-year cycle is to increase the probability to 0.43
Considering the fact that a non-leap year having 28days in February will definitely have 4 Tuesdays , we have to find the probability that the 29th day of a leap year (2008) is also a Tuesday. This probability is 1/7 i.e 1 out the probable 7days in a week ! Well this said, this question is void in a sense knowing that we are in 2010 and that February 2008 had 5 Fridays and therefore the probability of 5 Tuesdays is ZERO as of 2008 !