Sunday, 3 February 2008

Brain teaser 5 (solution)

You can learn the contents of all three boxes by drawing just one marble. The key to the solution is your knowledge that the labels on all three of the boxes are incorrect. You must draw a marble from the box labeled BW (black-white). Assume that the marble drawn is black. You know then that the other marble in this box must be black also, otherwise the label would be correct. Since you have now identified the box containing two black marbles, you can tell at once the contents of the box marked WW: you know it cannot contain two white marbles, because its label has to be wrong; it cannot contain two black marbles, for you have identified the box; therefore it must contain one black and one white marble. The third box, of course, must then be the one holding two white marbles. You can solve the puzzle by the same reasoning if the marble you draw from the BW box happens to be white instead of black.

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