Find the number

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  • debasisdas
    Recognized Expert Expert
    • Dec 2006
    • 8119

    Find the number

    A ten-digit number contains every digit from 0 to 9. The digits are
    arranged so that the number formed by the first two digits, reading from
    left to right, is divisible by 2, the number formed by the first 3 digits
    is divisible by 3, the first 4 by 4, and so on until the whole number is
    divisible by 10.

    can you please find the number ...!
  • Stang02GT
    Recognized Expert Top Contributor
    • Jun 2007
    • 1206

    #2
    im not gonna lie this ones tricky :}

    Comment

    • jkmyoung
      Recognized Expert Top Contributor
      • Mar 2006
      • 2057

      #3
      Let the 10 digits of the number be a-j, eg:
      abcdefghij
      abcdefghi0 div by 10
      abcd5fghi0 div by 5

      div by 2
      even digits must be in 2, 4, 6, 8, 0
      odd digits must be in 1, 3, 5, 7, 9.

      div by 3
      a+b+c mod 3 = 0
      d+e+f mod 3 = 0
      g+h+i mod 3 = 0

      div by 4
      Since c is odd, d must be 2 or 6.
      similarly, since g is odd, h must be 2 or 6.
      -> b and f are 4 and 8 in some order.

      div by 8
      Since f is 4 or 8, gh mod 8 = 0. Possibilities for gh:
      32, 72, 16, 96

      combined with div by 9 (3), the possibilities for ghi:
      321, 327, 723, 729, 963,

      d + 5 + f (mod 3) = 0
      d + f = 1 (mod 3)
      possibilities for def, given that f = 4 or 8 and e = 5
      654, 258

      do div by 7 seperately. So combining the options, and then taking mod of the first 7 digits we have:

      987654321 => 5 mod 7
      789654321 => 4 mod 7
      981654327 => 2 mod 7
      189654327 => 5 mod 7
      981654723 => 6 mod 7
      189654723 => 2 mod 7
      183654729 => 6 mod 7
      381654729 => 0 mod 7
      147258963 => 6 mod 7
      741258963 => 2 mod 7

      Answer is
      3816547290

      Comment

      • Stang02GT
        Recognized Expert Top Contributor
        • Jun 2007
        • 1206

        #4
        wow....very impressive....i dunno if i ever would have figured that out

        Comment

        • debasisdas
          Recognized Expert Expert
          • Dec 2006
          • 8119

          #5
          Originally posted by jkmyoung
          Answer is
          3816547290
          thanx for the explanation

          Comment

          • Person
            New Member
            • Jun 2007
            • 66

            #6
            [Edit]
            Sorry, didn't understand problem (don't know how to delete a post).

            Comment

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