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Imagine there are three closed boxes, each full of marbles on a table before you.
They're labeled "all blue marbles," "all red marbles," and "blue and red marbles."
You're told that the labels do describe the contents of the boxes, but all three labels are pasted on the wrong boxes.
You may reach into only one box blindfolded and remove only one marble.
Which box should you select from to enable you to correctly label the boxes?