Suppose there are two full bowls of cookies. bowl #1 has 10 chocolate chip and 30 plain cookies, while bowl #2 has 20 of each. our friend stacy picks a bowl at random, and then picks a cookie at random. we may assume there is no reason to believe stacy treats one bowl differently from another, likewise for the cookies. the cookie turns out to be a plain one. how probable is it that stacy picked it out of bowl #1?
