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Fake Gold Coins

Do not know if this one has been posted before, but here it goes ...

You have 100 gold coins, which are grouped in 10 piles of 10 coins.
One of the piles have 10 fake gold coins.

One pure gold coin weighs 100 grams and a fake gold coin weighs 99 grams.

You have an electronic scale, but the scale has only enough battery life to give one measurement.

So by only using the scale once, you must determine which pile of gold coins are fake.

Jean du Plessis
Wednesday, May 11, 2005

I like this one.  It's a bit of an Aha! but it requires you to puzzle out a non-normal solution to a problem when the inclination would be to do something completely different. 

Here's a solution:

Put 10 coins from the first group, 9 from the second... and 1 from the last group on the scale and take a measurement.

If all of the coins were real then the weight would be 5,500grams, but you'll be short.  The number of grams you're short by will point to the correct stack of coins.  For instance if you are short by 1 gram, then the pile you took one coin from is the fake stack.  If 10 grams are missing then the pile you took all 10 from is the fake stack, etc.

Wednesday, May 11, 2005

Well done, Lou , you are spot on!!!

Jean du Plessis
Thursday, May 12, 2005

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