# Sequence - stop at 90

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...

• Does this need some kind of knowledge-related tag?
– Stiv
Mar 17 '20 at 12:12
• No, not really. Its almost completely basic knowledge. Mar 17 '20 at 12:44

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.

• You got the correct rule! But not the correct missing two-digit number. How about the exception? Mar 17 '20 at 13:57
• another could be rot13(avargrra). Nice work. :) Mar 17 '20 at 13:59
• But still, why exactly these numbers? Alot more resoning needed! Mar 17 '20 at 14:08
• 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!
– Stiv
Mar 17 '20 at 15:06
• @Stiv, pretty sure you nailed it. Mar 17 '20 at 15:19