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Simple math

  1. May 17, 2007 #1
    how many 5 digit numbers are there in which every two neighbouring digits differ by 3?

    can you please tell me if i have to do this all by trial and error or is there some sort of formula i need to make to do this
  2. jcsd
  3. May 17, 2007 #2

    matt grime

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    Trial and error? You mean you're going to guess a number and see if it satisfies the condition?

    What if I told you the first digit were a 1? What is the second digit?
  4. May 17, 2007 #3
    but theres too many possibilites
  5. May 17, 2007 #4
    what i meant is do i need to make a formula to work out the number of combinations
  6. May 17, 2007 #5

    matt grime

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    There are not too many possibilities. And that isn't the only one that starts with 1. You could write down a recurrence relation, if you wished, but I doubt that will help - just do it, it isn't very hard, and won't take you very long.
  7. May 23, 2007 #6
    This was in the UNSW Maths comp. Hint: it is around 40...
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