KöMaL 2007 C905
Ann writes down two arbitrary natural numbers that consist of the same digits in different orders. She subtracts the smaller number from the greater one, and multiplies the difference by an arbitrary natural number. Then she deletes a non-zero digit from the product. She tells the remaining number to Bill. With a little thinking, Bill can guess the deleted digit. How?
Official text KöMaL. Quoted with source.
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