Number Sequence

[Joseph Bolas]

What number can replace 'X' in the following sequence, and why?

21, 19, 15, 11, 07, 04, 02, 06, 05, 03, 12, X, 18, 26,

NB: There are 2 possible answers, so I just require only one.

[Kirk Bevins]

I reckon it has something to do with the letters of the alphabet in code as there are only 1-26. However, if A is 1 and Z is 26 we have:

USOKGDBFECLRZ.

Which isn't a word.
We also have each letter is unique - UNCOPYRIGHTABLE is the longest word that fits this criterion but that's too long (and COPYRIGHTABLE is too short). I am on the right lines with letters Joseph?

[Joseph Bolas]

I reckon it has something to do with the letters of the alphabet in code as there are only 1-26.

I can tell you that if this sequence was extended, it would focus on 3 digit numbers and could even get to 4 digit numbers too (depending on how long the sequence went on for).

Am I on the right lines with letters Joseph?

Yes, but not in a A-Z = 1-26 code sense.

[Kirk Bevins]

How about in another code sense, such as an A=26, Z=1 code? Or is there no code?

[Joseph Bolas]

How about in another code sense, such as an A=26, Z=1 code? Or is there no code?

Nope, sorry . There is no code required to solve this.

[Gavin Chipper]

Looks like we're all struggling with this.

[Joseph Bolas]

Looks like we're all struggling with this.

Kirk has worked out its to do with letters, but if its not letters as in a code, how else can you relate those numbers to letters?

[M. George Quinn]

16!

Last clue gave it away, to me.

[Joseph Bolas]

Last clue gave it away, to me.

I was afraid that my clue might've gave it away, but I wasn't sure how to word it so it wasn't too much of a give away .

[Paul Howe]

And the other possible answer is 70?

[Joseph Bolas]

16!

Last clue gave it away, to me.

And the other possible answer is 70?

Yes. Both 16 and 70 fit the logic .

NB: Focusing on the numbers in the puzzle, there is obviously a 3rd number, but I was looking for the 2 not already there .

[Gavin Chipper]

What about 3π?

(Sorry)

[Joseph Bolas]

What about 3π?

(Sorry)

[Dinos Sfyris]

I still dont get it and now feel incredibly thick. Could somebody please explain why the answer is 16 or 70?

[Paul Howe]

Dinos, try writing the numbers out in words.

[Dinos Sfyris]

Oh now I get it! I feel even more thick for not seeing it before!

[Joseph Bolas]

Does anyone know if Kirk got this in the end?

[Charlie Reams]

I imagine Kirk does.

[Kirk Bevins]

Does anyone know if Kirk got this in the end?

No I haven't got it - and still can't get it. I've tried writing them out in words in my head and they're not alphabetical. I have no idea. Tell us the answer please, Joseph, I've been waiting 9 months.

[Joseph Bolas]

No I haven't got it - and still can't get it. I've tried writing them out in words in my head and they're not alphabetical. I have no idea. Tell us the answer please, Joseph, I've been waiting 9 months.

Sorry about that Kirk

If you write the numbers out in word form, the number of letters (excluding spaces) from 21 - 02 in the first half of the sequence go down by one each time and the number of letters (excluding spaces) from 06 - 26 in the second half of the sequence go up by one each time.

To fill in 'X', you needed to come up with a 7-letter number and the two answers you could've had (that wasn't mentioned already in the puzzle) were SIXTEEN and SEVENTY

21 - TWENTYONE - 9 Letters
19 - NINETEEN - 8 Letters
15 - FIFTEEN - 7 Letters
11 - ELEVEN - 6 Letters
07 - SEVEN - 5 Letters
04 - FOUR - 4 Letters
02 - TWO - 3 Letters
06 - SIX - 3 Letters
05 - FIVE - 4 Letters
03 - THREE - 5 Letters
12 - TWELVE - 6 Letters
X
18 - EIGHTEEN - 8 Letters
26 - TWENTYSIX - 9 Letters

[Kirk Bevins]

So SIX and TWO are interchangeable as are FOUR and FIVE? What a rubbish sequence. Thanks for telling me the answer though.