(adapted from ross, 4.60) suppose that the number of times that vampire bill gets attacked by hep v in a given year is a poisson random variable with parameter λ = 5. now, he discovered a new drug (based on large quantities of vitamin c) that has just been marketed to protect from hep v. it claims to reduce the poisson parameter to λ = 3 for 75% of the population. for the rest of the population, the drug has no appreciable effect on hep v. if bill tries the drug for a year and only had 2 hep v attacks in that time, how likely is it that the drug was beneficial for him?