1. The problem statement, all variables and given/known data The time between calls to a corporate office is exponentially distributed with a mean of 10 minutes. What is the probability that there are more than three calls in one-half hour? 2. Relevant equations F(x) = P(X <= x) = 1 - e^-(lamba*x) 3. The attempt at a solution P(X > 3) = 1 - P(X < 3) = 1 - [1 - e^-(10*3)] =e^-(10*3) I think this answer is wrong though because I think that P(X > 3) actually means the probability that at least 30 minutes will pass before the first call instead of the probability that there are more than three calls in one half hour, but I'm not sure.