I have 3 jars. All of the jars are incorrectly labeled. One has peaches, one has pears, one has plums.
What is the least number of jars I need to open to determine the contents of all 3 and why?
Only one.
You open the one labeled peaches. It cannot contain peaches. There are two cases, either it contains pears, or plums.
  1. You found plums in the jar labeled peaches. Since the remaining jar labeled pears can't contain the pears, the pears must be in the jar labeled plums.
  2. You found pears in the jar labeled peaches. Since the remaining jar labeled plums can't contain plums, the plums must be in the jar labeled pears.
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If the jars are transparent you don’t have to open any of them
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