we have 2 coins: coin 1 comes up with the probability of tails of 2/3 while coin 2 comes up with the probability of tails of 1/3 you don't know which is which so you pick one up at random and flip it . If its tails, you guess that the coin you picked up is coin 1 else you guess that the coin you picked up is coin 2. what is the probability that your guess is correct.