Enigma 1589: Coin-cidence
23 January 2012
Posted by on
From New Scientist #2754, 3rd April 2010 [link]
I recently purchased a number of seemingly identical antique coins knowing that an unspecified one of them was counterfeit and significantly different in weight from the others.
By coincidence, I have another exactly similar and authentic coin already in my collection.
Using a sensitive two-pan balance, I calculate that to be certain of finding the offending coin and whether it is heavier or lighter than the others, I shall need three weighings.
Had the number of coins I purchased been any larger, I would have needed at least one more weighing to be sure of achieving the same outcome.
How many coins did I purchase?