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I heard this puzzle in a quiz I participated in:

10, 11, 12, 13, 14, 15, 16, 17, 20, 22, 24, ?, 100, 121, 10000

What is the missing number?

(You don't need to do any big calculations, and you don't need any advanced mathematical knowledge - just one basic concept that's known to many non-mathematical people.)

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  • $\begingroup$ What happened to @d'alar'cop 's answer? It wasn't what I was looking for, but sounded like it could be an interesting alternative! $\endgroup$
    – Rand al'Thor
    Nov 14, 2014 at 14:26
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    $\begingroup$ Possible duplicate: math.stackexchange.com/questions/584592/… $\endgroup$
    – Kenshin
    Nov 14, 2014 at 14:31
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    $\begingroup$ This question appears to be off-topic because it is about Maths and has an exact duplicate in that site $\endgroup$
    – skv
    Nov 14, 2014 at 14:42
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    $\begingroup$ @skv - It's still a puzzle, even if it's a maths puzzle. Didn't know about the duplicate though. $\endgroup$
    – Rand al'Thor
    Nov 14, 2014 at 14:44
  • $\begingroup$ @randal'thor I agree, I am guilty of posting pure math questions here earlier too, but unless its disguised in words, its tough to justify that they are more than calculations, and hence should be discouraged on this site, I am sure you will understand $\endgroup$
    – skv
    Nov 14, 2014 at 15:27

1 Answer 1

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According the google the answer be 31.

The sequence is 16 in base 16-n.

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    $\begingroup$ oeis.org/A008713 $\endgroup$
    – Set Big O
    Nov 14, 2014 at 14:19
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    $\begingroup$ @Geobits: using OEIS is cheating! :-( $\endgroup$
    – Rand al'Thor
    Nov 14, 2014 at 14:19
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    $\begingroup$ @randal'thor, using my intelligence. $\endgroup$
    – Kenshin
    Nov 14, 2014 at 14:22
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    $\begingroup$ "just one basic concept that's known to many non-mathematical people" .. oof, I wonder how many people who aren't that into maths know about bases and especially its conversions $\endgroup$
    – No. 7892142
    Nov 14, 2014 at 14:25
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    $\begingroup$ @Mew - Google is almost as cheating as OEIS! (unless you were joking) $\endgroup$
    – Rand al'Thor
    Nov 14, 2014 at 14:28

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