Edmund's Brainteasers
Problem 23
Problem 23: The All Saints raffle
Ian wins the All Saints raffle but is surprised by the prize. He is presented with three identical wine boxes: he is told that one contains six bottles of a particularly pleasing red, while the other two boxes are empty. He chooses one at random. Then Jonathan, who knows which box contains the wine, opens one of the other two boxes showing it to be empty. He offers Ian the choice of sticking with the box of his first choice or changing to the other unopened box. Would Ian improve his chances of winning the wine by swapping, or is he as well to stick with his first choice?