A keypad is a by grid with the digits to in order on the top row, to in order on the second row and to in order on the third row.
Fred has the keypad upside-down (that is, rotated through ) and types his four-digit PIN as if the keypad were the right way up. The number he types in turns out to be more than times his original PIN where is a positive integer. Prove there is only one possible PIN and find it.
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Hamilton Mathematical Olympiad (2025)
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