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Can you solve the counterfeit coin riddle? - Jennifer Lu

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You’re the realm’s greatest mathematician, but ever since you criticized the Emperor’s tax laws, you’ve been locked in the dungeon. Luckily for you, one of the Emperor’s governors has been convicted of paying his taxes with a counterfeit coin, which has made its way into the treasury. Can you earn your freedom by finding the fake? Jennifer Lu shows how.


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About TED-Ed Animations

TED-Ed Animations feature the words and ideas of educators brought to life by professional animators. Are you an educator or animator interested in creating a TED-Ed Animation? Nominate yourself here »

Meet The Creators

  • Educator Jennifer Lu
  • Director Outis
  • Script Editor Alex Gendler
  • Associate Producer Jessica Ruby
  • Content Producer Gerta Xhelo
  • Editorial Producer Alex Rosenthal
  • Narrator Addison Anderson
Additional Resources for you to Explore
The counterfeit coin riddle is derived from the mathematics field of deduction, where conclusions are systematically drawn from the results of prior observations. This version of the classic riddle involves 12 coins, but popular variations can consist of 12 marbles or balls. Part of the appeal of this riddle is in the ease with which we can decrease or increase its complexity. A simple swapping of a few key words drastically alters the problem’s difficulty.

To make the problem simpler, for example, we can reduce the total number of coins to 9. To further simplify, we can reveal whether the counterfeit is lighter or heavier at the onset, as is shown here. For a harder version of the problem, start with 39 coins and four weighs.

In all cases, the counterfeit coin riddle offers a fun introduction into the world of deductive reasoning.

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Create and share a new lesson based on this one.

About TED-Ed Animations

TED-Ed Animations feature the words and ideas of educators brought to life by professional animators. Are you an educator or animator interested in creating a TED-Ed Animation? Nominate yourself here »

Meet The Creators

  • Educator Jennifer Lu
  • Director Outis
  • Script Editor Alex Gendler
  • Associate Producer Jessica Ruby
  • Content Producer Gerta Xhelo
  • Editorial Producer Alex Rosenthal
  • Narrator Addison Anderson