Question 158
You take a wrong turn on the way to MIT and end up in Transylvania, where $99\%$ of the inhabitants are vampires and the rest are regular humans. For obvious reasons, you want to be able to figure out who's who. On average, nine-tenths of the vampires are correctly identified as vampires and nine-tenths of humans are correctly identified as humans. What is the probability that someone identified as a human is actually a human?