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We start from the string $ABCDEF$ and we can edit it with the following rules:

1.$ABCDEF \Rightarrow ADBECF$

2.$ABCDEF \Rightarrow DAEBFC$

Which string can't be accessed?

  1. DBAFEC
  2. FCBEDA
  3. CABEFD
  4. EFDCAB
  5. FEDCBA

In these kind of problems we usually look for a common thing in every change of the the string, which I can't do here.

According to the book, the answer is

string $5$.

Math SE copy: https://math.stackexchange.com/questions/2444884/which-string-cant-accessed

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  • 2
    $\begingroup$ For content that is not your own, please provide attribution. Using someone else's content without (at minimum) disclosing where it came from is plagiarism, and such posts are deleted. $\endgroup$ – Rubio Sep 25 '17 at 18:22
  • $\begingroup$ Not to mention cross posting is generally discouraged without good reason. $\endgroup$ – Bobson Sep 25 '17 at 20:21