[*]You're standing outside the nursery in the maternity ward of a hospital. You can't see in, but you know that there is exactly one boy and some unknown number of girls inside. Just then, a nurse walks into the nursery with another newborn, but you can't tell if it's a boy or a girl. A man, following close behind and grinning from ear to ear, points at the baby and exclaims, "Hi, I'm Bob, and that's my first child!" He then continues on down the hall sharing the good news with everyone he meets. A few minutes later, a baby is randomly selected from the nursery, and it is a boy. What is the probability that Bob's child is a girl?
[*]You have a bag containing 100 marbles, each of which is equally likely to be either white or black. At the time the bag was filled, every possible configuration of white and black (e.g. 50 whites/50 blacks, 100 whites/0 blacks, 73 whites/27 blacks) was equally likely. One at a time, you reach in and randomly select one, note its color, and return it to the bag. You do this 100 times and all 100 times you happen to pull a white marble. What is the probability that the bag contains 100 white marbles and no black marbles?
[*]An acquaintance offers to play a game where you bet on the outcome of a flipped coin. If it lands heads, you have to pay him a dollar; if it lands tails, he pays you $X. Sounds good, but you happen to know that he has two coins in his pocket, one of which is a normal fair coin and one of which is a trick coin with heads on both sides. You will play the game with one of these coins, but neither of you is sure which one (he will just randomly pull one out and start flipping).
2) If he pulls out one of the coins, flips it four times, and it comes up heads all four times, what is the probability that you're playing with the trick coin?