My friend Josh shared a riddle he got from an MIT-grad coworkers. I tried to figure it out for a while, but I quickly realized that I ended up thinking myself in a circle.
Here’s the deal:
- You have 13 stacks of 4 coins.
- 12 stacks are legitimate coins, one stack is counterfeit coins.
- There is no discernible difference between counterfeit coins and legitimate coins aside from their weights.
- All legitimate coins weigh the same amount. All counterfeit coins weigh the same amount.
- The weight of a legitimate coin is an integer.
- The weight of a counterfeit coin is within 5 grams of the weight of a legitimate coin.
Using a scale that tells you (with as much precision as you need) the weight of anything placed on it, with 2 weighings:
- Can you find the stack of counterfeit coins?
- Is it possible to determine the weights of both a counterfeit coin and a legitimate coin?
I’m beyond the “I’ll just look up the answer on the Internet,” and I’m not entirely sure that all of the details are included here … I’m going off what I believe is still good memory.
I’ve been told that the answer doesn’t involve any wordplay or trickery (like “The ancient Romans used the term ‘2 weighings’ casually to mean ‘by using a scale that tells you which coin is counterfeit’”).
Since there are a few similar (but different) coin-related riddles making around the Internet, I have yet to find anyone to spoil this for me. Can you? The “Let People Answer This” option won’t give enough space for the full answer (If it does, I’d feel like an idot), so reblog if you know the answer … or if you think one of your friends can figure the answer out.
