Chapter 1: Problem 18
A secretary types three letters and the three corresponding envelopes. In a hurry, he places at random one letter in each envelope. What is the probability that at least one letter is in the correct envelope? Hint: Let \(C_{i}\) be the event that the ith letter is in the correct envelope. Expand \(P\left(C_{1} \cup C_{2} \cup C_{3}\right)\) to determine the probability.