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Problem of the Week 1103
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Alice: Pick a 5-digit number.
Bob: Got it.
Alice: Now reverse it and subtract it from the original.
Alice: Now pick one of the five digits of the result and circle it (but don't choose a zero). Tell me the other four digits, in some random order.
Bob: Sure; they are 1, 1, 0, and 8.
Alice: Then the circled digit is....
Alice does indeed correctly give the circled digit. How did she do it?
Suggested by Bill Owens, Macalester '01.
© Copyright 2008 Stan Wagon. Reproduced with permission.
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15 September 2008