Three closed boxes have either white marbles, black marbles or both, and they are labeled white, black and both. However, you’re told that each of the labels are wrong. You may reach into one of the boxes and pull out only one marble. Which box should you remove a marble from to determine the contents of all three boxes?
December 1st, 2011 | Category: 
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Both. If you know that the ‘both’ box does not actually contain both colors, then the color you pull out will be the only color in the box. Say that you pulled a white marble from the ‘both’ box – now you know that the box labeled ‘white’ contains either black or both colors, and the box labeled ‘black’ contains either black or both. Since we know that the label on the black box is wrong, it must contain both colors. By deduction then, the ‘white’ box contains black marbles.
Correct, Rachel.
Couldn’t have explained it better.
You’re today’s winner.