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This is an easy number puzzle:

1) 1
2) 11
3) 21
4) 1211
5) 111221
6) 312211
7) 13112221
8) ???

What is the number on row 8?

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  • $\begingroup$ Didn't see it's been already posted, sorry guys $\endgroup$ – Alex Mar 19 '15 at 17:19
  • $\begingroup$ Eh, this one was a little hard to find its dupe, so no biggie. $\endgroup$ – JonTheMon Mar 19 '15 at 18:25
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It is:

1113213211

Explanation:

Each number, from the second line onwards, represents the frequencies of the digits in the previous line in the form $Af_ABf_BCf_C...$, where $A$, $B$ and $C$ are the different digits in order of appearence, and $f_{A,B,C}$ represents their frequency in every continuous set of equal digits.

1
11      one one
21      two ones
1211    one two and one one
111221  one one, one two, and two ones
...
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  • $\begingroup$ Correct guess but careless answer~ $\endgroup$ – Alex Mar 19 '15 at 17:19
  • $\begingroup$ @Alex I edited my answer adding a more precise explaination $\endgroup$ – Marco Bonelli Mar 19 '15 at 17:32

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