If license numbers consist of 3 letters followed by 3 digits, how many different licenses could be created having at least one letter or digit repeated?

Answer 1

Number of all possible plates
N1=26³*10³

Number of plates with unique letters AND unique digits
N2=26*25*24*10*9*8

The difference of N1 and N2 gives the number of plates with at least one repeated digit or letter.