1. The problem statement, all variables and given/known data On the average, a grocer sells 4 of a certain article per week. How many of these should he have in stock so that the chance of his running of stock within a week will be less than 0.01? Assume Poisson distribution. 2. Relevant equations 3. The attempt at a solution So I set λ = 4, plugged it into e^(-λ)λ^(x) / x! set it <0.01 and looked for an x that brought the equation to <0.01. I was unsure how solve this for x, because of the x! in the bottom, so I just started with x = 0 and plugged and chugged till I came across x = 10, P(X=10)=0.0053 < 0.01. However this is incorrect. Any suggestions?