This question already has an answer here:
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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marked as duplicate by Kevin, singletee, Community♦ Mar 19 '15 at 17:19
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
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$\begingroup$ Didn't see it's been already posted, sorry guys $\endgroup$ – Alex Mar 19 '15 at 17:19
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$\begingroup$ 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
...
answered Mar 19 '15 at 17:15
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$\begingroup$ @Alex I edited my answer adding a more precise explaination $\endgroup$ – Marco Bonelli Mar 19 '15 at 17:32
