The answer is 15. Just count the spaces between A and D.
        
             
        
        
        
There are 26 letters of the alphabet
you want 6-character passwords without repetitionthe first time you pick a letter, you pick from 26 lettersthe second time you pick a letter, you pick from 25 letters (no repetitions !)the third time you pick a letter, you pick from 24 letters, and so onthe answer is 26*25*24*23*22*21=165,765,600 passwords
        
             
        
        
        
Same solution.
volume = 1/3(8x6)(5) = 16x5 = 80cm^3
answer: 80cm^3