You start with 0 and then proceed as follows:

1, 8, 2, 1, 11, 9, 18, ??, 90

  • There's no other number after 90 in this sequence. In fact, it's impossible to find!

  • One of the numbers can be replaced by another number, but this sequence would take up alot more space if so, so I made an exception and made it simpler.

So, what is the rule of this sequence? What number is the exception(why)? And what two-digit number is missing?

EDIT: Made a mistake in the sequence! It's correct now. Guess, it will be easier now...

  • $\begingroup$ Does this need some kind of knowledge-related tag? $\endgroup$
    – Stiv
    Mar 17, 2020 at 12:12
  • $\begingroup$ No, not really. Its almost completely basic knowledge. $\endgroup$ Mar 17, 2020 at 12:44

1 Answer 1


The rule is:

All numbers begin with the letter the previous one ends with

Thanks to @Stiv, we also know that:

The number chosen is the lowest 1 or 2-digit number beginning with that letter that isn't already in the sequence.

Starting with 0:

Zero > One > Eight > Two > One > Eleven > Nine > Eighteen > Nineteen > Ninety

You can't go further because

no number begins with y

This may not be all there is to it though.

  • $\begingroup$ You got the correct rule! But not the correct missing two-digit number. How about the exception? $\endgroup$ Mar 17, 2020 at 13:57
  • 1
    $\begingroup$ another could be rot13(avargrra). Nice work. :) $\endgroup$
    – MacGyver88
    Mar 17, 2020 at 13:59
  • 1
    $\begingroup$ But still, why exactly these numbers? Alot more resoning needed! $\endgroup$ Mar 17, 2020 at 14:08
  • 2
    $\begingroup$ Like @MacGyver88 says your missing number is probably rot13(avargrra - vg'f gur YBJRFG ahzore bs srjre guna 3 qvtvgf ortvaavat jvgu gur ynfg yrggre bs gur cerivbhf, hfvat ahzoref guhf-sne-hahfrq vs ninvynoyr. 1 vf ercrngrq orpnhfr vg vf gur bayl bar ortvaavat jvgu B. V'z thrffvat gur BC yvzvgrq gur ehyr gb ahzoref bs srjre guna 3 qvtvgf, ryfr gurl'q unir unq bar-uhaqerq nsgre gur 2, juvpu jbhyq grezvangr gur frdhrapr noehcgyl, fvapr ab ybj ahzoref ortva jvgu Q.) Not sure about the exception - this rule seems pretty solid unless I've missed something amongst its constraints! $\endgroup$
    – Stiv
    Mar 17, 2020 at 15:06
  • $\begingroup$ @Stiv, pretty sure you nailed it. $\endgroup$
    – MacGyver88
    Mar 17, 2020 at 15:19

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