An n- bit string is n- digit word , each digit of which is either 0 or 1.

Find the no. of n- bit strings containing both the digit zero and one.

Dear Sanchit

Each position of n digit string can be filled by 2 ways either 0 or 1

so total number os such word is 

     =2*2*2*2*2 .......n times

      =2ⁿ

 but it include that case also which has 0 at all position and 1 at all position

So total number of desired way =2ⁿ-2