Question 134446
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Draw one from the bin labeled "mixed".  Re-label it with that same color.
Relabel the bin originally labeled with that color with the opposite color,
and re-label the remaining bin "mixed".
 
Explanation:

You know that the bin labeled "mixed" contains only marbles of the
same color, since it's incorrectly labeled.  

So draw one marble from the bin labeled "mixed".  If it is white then
it contains only white marbles, so re-label it correctly as "white".

Then the bin originally labeled "black" cannot contain only black 
marbles since it is labeled incorrectly.  And it certainly can't 
contain only white marbles because you have just finished correctly 
re-labeling the all-white bin!  So it can only be the bin with mixed 
colored marbles. 

So the remaining bin, which was originally labeled "black" contains
only white marbles.

It's exactly the same explanation if you drew a black marble, except 
you interchange the words "white" and "black".

Edwin</pre>