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Exercise 2.

 Suppose that a room contains n people. What is the probability that at least two of them have a birthday on same month? Make a plot of probability vs number of people and find out for how many people does the probability becomes greater than 0.5 and 0.9. 

Sixty percent of the students at a certain school wear neither a ring nor a necklace. Twenty percent wear a ring and 30 percent wear a necklace. If one of the students is chosen randomly, what is the probability that this student is wearing