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One day a prison guard noticed a series of numbers scratched on the wall of an inmate's cell. Concerned it was something to do with an escape plan, the inmate was immediately questioned.


(the question marks are part of the riddle)

Evidence Photo


Guard: What do those numbers mean?

Inmate: I just moved into this cell last night, the previous inmate died.

It was clear based on the age of the scratches, the inmate was telling the truth.

The numbers became a sort of legend in the prison for some time to come. Everyone tried to figure out what they meant, but only one inmate truly figured it out.

The inmate who claimed to possess the answer to the riddle was in court one day. The judge was fascinated with the number sequence and what they meant. He had to know. So he made the inmate a peculiar, yet tempting offer:

The inmate could take a one year sentence for his crime or divulge the meaning of the numbers and/or question marks and go home.

The Verdict

The inmate spoke 3 words, followed by an explanation of those words, and was immediately released.

What did the inmate say, and why?

  • 9
    $\begingroup$ "It is gibberish." $\endgroup$ – user118161 Apr 12 '20 at 1:10
  • 2
    $\begingroup$ HINT in rot13: V jvyy tvir lbh n uvag. Guvax cevzr ahzoref. $\endgroup$ – John S. Apr 12 '20 at 2:11
  • $\begingroup$ @ShivPrateek I would suggest putting your observations in rot13, as to not spoil the puzzle for other viewers. Also, rot13(Gbbx zr n juvyr gb ernyvmr gur aba-mreb ahzoref pbeerfcbaq gb gur ahzoref’ erfcrpgvir cbfvgvbaf sebz gur yrsg-unaq fvqr.) $\endgroup$ – Cloudy7 Apr 12 '20 at 5:12
  • $\begingroup$ Are they rot13(cevzr zvahf guerr)? $\endgroup$ – newbie Apr 12 '20 at 10:21
  • $\begingroup$ @newbie !!! Terrific job! Yes. Please POST YOUR ANSWER so I can accept. $\endgroup$ – John S. Apr 12 '20 at 17:56

My guess:

The non-zero entries are primes minus three. $\{2,4,8,10\}=\{5,7,11,13\}-3$. And these numbers are put into corresponding places.

Why a pattern like this,

Maybe the prisoner is trying to do a Sieve of Eratosthenes?

  • $\begingroup$ The comments and solutions are so, so close. ** ROT 13 SPOILER ** Npghnyyl, nyy cevzr ahzoref naq ahzoref gung pna or qvivqrq jvgu 3 unir orra ercynprq jvgu n mreb. Gur arkg 3 ahzoref jbhyq or mreb, mreb, naq mreb $\endgroup$ – John S. Apr 13 '20 at 17:02
  • $\begingroup$ I guess I didn't really get it... I'll leave it here. @JohnS. $\endgroup$ – newbie Apr 14 '20 at 3:03

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