Hi, 1. The problem statement, all variables and given/known data I am somewhat perplexed by the proposed solution to the following Statistics problem and was hoping one of you might be willing to help me settle this: An operator receives phone calls between 8AM and 4PM at an average rate of 20 calls/hour. No call was received during the first 3 minutes. What is the probability that we shall have to wait at least one more minute before the first call is received? 2. Relevant equations 3. The attempt at a solution Now, the book states it ought to be P(T≥1) = 1 - FT(1) = e-1/3. My question is, why should it not have been 1 - [F(4) - F(3)], i.e. the complement of a call received between the 3rd and 4th minutes? I'd appreciate it if any of you could explain this to me.