Answer
Method of Proof: Proof by Contradiction (Reductio ad Absurdum)
Work Step by Step
To prove: That at least three of any 25 days chosen must fall
in the same month of the year.
Hypothesis: Let us assume the negation of this statement, i.e there is no month in which 3 or more dates fall.
So each of the 12 months can have at most 2 dates in it. But that accounts for at most $ 12 \times 2 =24$ months and we are given that there are 25 days.
Thus, our hypothesis must be false and there indeed is a month in which at least 3 days fall.
Hence proved.
