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The Brainliest Answer!
2016-01-03T19:38:32+05:30
In this series 1 we spell it 1 so, get  11 now, 11 we spell it two one so we gets 21,now 21 we spell it one 2 and one 1 so we gets 21 as common and two one that is 1211
1 5 1