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Number Tricks - Bonus Question

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Question 4 - Bonus

This trick is based on the number of matches in a matchbook.

  1. Take an unused matchbook that contains 20 matches and tear out any number of matches, up to 9.

  2. Throw them away and count the matches that remain.

  3. Add the 2 digits of this number and tear out the same number of matches from the book.

  4. Tear out 2 more matches

Try this a couple of times. What is the result?

Think of a way to prove the answer will always be the same. You do not need to use algebra if you can find another method.

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Please mail comments and suggestions to Richard Briston
28 January 1996