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Cake day: June 14th, 2025

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  • It couldn’t be row A or B because there would then be a possibility the ball was in A6 or B5 and that Bernard actually did know where the ball was. Albert says he knows Bernard doesn’t know where the ball is, which means he is 100% confident, and the only rows that allow for that confidence are rows C and D, because each ball in each row is not alone in its column.

    Scenario 1: The madman tells you the ball is in row A. Then the madman whispers the column to Bernard. Can you really be sure that Bernard knows or doesn’t know where the ball is?

    Scenario 2: The madman tells you the ball is in row C. From this, you can be completely sure that Bernard doesn’t know where the ball is, because any column he would be given doesn’t narrow it to a single row.



  • C3. I used to do these logic puzzles at work.

    Both Albert and Bernard know the ball is at a certain row and at a certain column, respectively. Albert first admits two things:

    1. He doesn’t know where the ball is. Aka, the row containing the ball has more than one ball in it. This isn’t important now, but will be re-used when he asserts it again.
    2. Bernard doesn’t know either. If Bernard knew where the ball was simply from the column, it’d be because that column only had one ball in it. Since he knows that Bernard doesn’t know given just the row, each ball in that row is in a column that contains more than one ball.

    This eliminates rows A and B, since B5 and A6 are the only balls in their columns. For this to work, Bernard now has to understand the above.

    Then Bernard admits that now he knows where the ball is. Since we can eliminate or ignore each ball in A and B, it can either be balls D2, C3, and D4 and not any ball in column 1 using logic from #2.

    At this point, Albert knows whether the ball is in C or D. If the madman told him the ball was in row C, then he would instantly know the ball is C3 given the elimination of C1 and D1. If he was told it was row D, then he still wouldn’t know.

    However, Albert admits the former, which tells us it’s C3.