Wednesday, August 15, 2007

White and Black

In a box you have 13 white marbles and 15 black marbles. You also have 28 black marbles outside the box. Remove two marbles, randomly, from the box. If they are of different colors, put the white one back in the box. If they are the same color, take them out and put a black marble back in the box. Continue this until only one marble remains in the box. What color is the last marble?


  1. The last marble is white b/c there is an odd number of white marbles, and you can never pair all of the white marbles with each other (due to the odd number issue). Since the only way to remove a white marble is to remove two at a time, you will be left with a white marble as the last man standing.

  2. As anonymous points out, the last marble will always be white. Since marbles can only be taken out in pairs and white has an odd number, one white marble will be left in the end.


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