by John Owen
Published September 10 2017 (link)
Three expert logicians played a game with a set of twenty-one cards each containing a different two-figure prime number. Each drew a card and held it up so that they could not see their own card but could see the others. Alf, Bert and Charlie in turn were then asked two questions, namely “Is your number the smallest of the three?” and “Is your number the largest of the three?”. In the first round all three answered “Don’t know” to both questions. The same happened in rounds two and three. In round four Alf answered “Don’t know” to the first question.
What did Alf answer to the second question and what numbers did Bert and Charlie have?