1. The problem statement, all variables and given/known data Accidents at a busy intersection follow a Poisson distribution with three accidents expected in a week. What is the probability that at least 10 days pass between accidents? 2. Relevant equations F(X) = 1- e-λx μ = 1/λ 3. The attempt at a solution Let x = amount of time between accidents in days My r.v. is continuous so x~Exponential(λ=?) E(x) = 3/7 (in days) Since E(x) = μ = 1/λ = 3/7 λ = 7/3 Thus x~Exponential(λ=7/3) P(X≥10) = 1-P(X≤10) = 1- F(10) = 1- e-70/3 Answer in the back of the book says: λ = 3/7 P(X≥10) = 1-P(X≤10) = 1- F(10) = 1- e-30/7 I'm confused why λ = 3/7 and not 7/3 if my expected value is 3/7. Shouldn't lambda, by definition, be its reciprocal?