Level
2

The answer is 6210001000.

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Let there be n number of zeroes so (n+1)th digit is 1. Now 2nd digit cannot be 1 as then number of 1's would be 2 so 2nd digit would be 2. This means 3rd digit is 1(as there is a single 2) so our number looks like : n21 followed by all zeroes except the (n+1)th digit which is 1. the number of zeroes can now easily be counted to be 6 and hence the 7th digit is 1. the solution thus being 6210001000