The number of n digit numbers which consists of the digits 1 & 2 only if each digit to be used at least once, is equal to 510 then n  is equal to _________

Basically we have n  blank spaces in which we have to put 1 and 2
 Total such occurrence =  $2^n$
But we have to subtract the cases where there is no occurrence of 1 and no occurrence of 2.
Hence, Number with at least on occurrence =  $2^n-1-1=2^n-2$
 $2^n-2=510\Rightarrow n=9$
Hence correct answer is 9.