Suppose someone tossed 100 coins and then covered them up. They ask you how many heads you expect, and absent other info you say 50. Suppose now that this person uncovers 20 of the 100 coins and shows you 20 heads. Note that they don't randomly uncover 20 coins which happen to be all heads - they purposefully show you 20 heads. So it's as if we have a crystal ball that gives us new info which is, "there are at least 20 heads out of the original 100 coin tosses". Do you change your estimate of how many heads in total and how? I'm having some trouble constructing Bayes rule for this particular example - thanks for your help!