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:: Gold and Copper Coins ::

There are 10 open boxes containg 100 coins each. In 9 of these boxes the coins are made of gold, and in the other the coins are made of copper. You are given a large digital balance which can be used once only. Can you identify the box containg copper coins knowing the weight of both gold and copper coins?

Answer: Take 1 coin from the first box, 2 from the second, 3 from the third box, etc. and weight all of them at once. Since there are 55 coins in total and the total weight of these coins will be less than that of 55 gold coins, the weight difference of these coins divided by the weight difference of one gold and one copper coin will give the box number that contains the copper coins.

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