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Puzzle of the Day

These puzzles are also printed in Eight days a week, a booklet published by the Association of Teachers of Mathematics.

Start with a two-digit number, reverse the digits to make a new number then subtract the smaller from the larger number. Repeat this process until you reach a single digit. Start with other numbers. What do you notice? e.g. 82-28=54, 54-45=9

Answer: Always ends at 9