Question

Find the next number in the series 1, 11, 21, 1121, ??

Answer 1

111221

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Everi number is based on previous one with the logic of counting numbers as following:
1 = 1x1 -> 11
11 = 2x1 -> 21
21 = 1x2, 1x1 -> 1211
1211 = 1x1, 1x2, 2x1 ->111221