Hosted by The Math Forum## Problem of the Week 1245## Goldilocs
Charlie has 128 identical-looking gold-colored coins; all are counterfeit except one.
He calls Alice into his office and seats her at a table containing two He tells Alice which coin is made of gold. Alice can then turn at most 4 coins upside down, replacing them on their squares, but her inversions must all be on the first row of the left-hand board. She then leaves the room. Bob enters and is seated in the same position, facing the boards. He may take one of the 128 coins. Find a strategy that allows Bob to always take the gold coin. As usual, Alice and Bob know the protocol in advance and can plan a strategy, but cannot communicate after Alice enters the room.
Note: If Alice could invert up to 7 coins, then she could arrange the first 7 squares of the left-board's first row to be any of Source: Peter Saltzman, Jim Tilley, and Stan Wagon. ("Goldilocs" abbreviates "Gold Locations.") |

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21 July 2017