Coin-cidence
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.
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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?
WIN 拢15 will be awarded to the sender of the first correct answer opened on Wednesday 5 May. The Editor鈥檚 decision is final. Please send entries to Enigma 1589, New 杏吧原创, Lacon House, 84 Theobald鈥檚 Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address).
Answer to 1583 Have a good day: the dates were 24 March and 26 July
The winner Paul Stillman of Wokingham, Berkshire, UK