I am told at the beginning of a mushroom-hunter’s guide that 28 of the 96 species described are good to eat, but the guide is not organized that way. I want to learn about edible mushrooms, so I...

I am told at the beginning of a mushroom-hunter’s guide that 28 of the 96 species described are good to eat, but the guide is not organized that way. I want to learn about edible mushrooms, so I decide that on my first day of study, I will read about species at random until I have read articles about 3 edible species.

a. What is the probability that I will have read about at most 4 inedible species?

b. Now redo the calculation, using the negative binomial approximation. Is that approximation plausible here? How close is your result to the exact answer?