1. The problem statement, all variables and given/known data A certain delivery service offers both express and standard delivery. Eighty-five percent of parcels are sent by standard delivery and 15% are sent by express. Of those sent standard, 80% arrive the next day, and of those sent express, 95% arrive the next day. A record of parcel delivery is chosen at random from the company’s files. Given that the package did not arrive the next day, what is the probability that is was sent standard? 2. Relevant equations Not entirely sure, possibly the statistics probability rule of multiplication or another. 3. The attempt at a solution To begin I made Express Delivery= to the variable "E" Standard delivery = "S" Next day = "N" From here I tried to use the relation P(S|N^c)=(P(N^c|S)P(S))/(P(N^c)) but the P(N^c|S) is one term that I don't have an actual probability for. So I'm going back and forth with one equation and two unknowns. I know the numerical value of the answer already, but I'm trying to follow the steps for the solution to the above listed problem. If someone could show me what the setup is for this solution I'd really appreciate the help.