Question

You have 80 coins that are identical in appearance. Among them there are three fake coins and you know which ones. The genuine coins all have the same weight and the fake coins all have the same weight, but the fake coins are lighter than the real ones. Your friend knows that among the 80 coins there are either three or two fake coins. Without revealing the identity of any of the 80 coins, how can you use a pan balance to convince your friend that there are exactly three fake coins?

Answer 1

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