Edmund's Brainteasers
Problem 23
Solution
Problem 23: The All Saints raffle. Solution.
Ian's first choice has a 1 in 3 chance of being right. There is a 2 in 3 chance that the wine is in one of the other boxes. However, if the wine is in one of the other 2 boxes, he now knows which one since Jonathan has shown one of these to be empty. If he swaps, he therefore has a 2 in 3 chance of being right while only a 1 in 3 chance if he sticks with his original choice. Swapping doubles his chances of winning the wine.