License plates in a particular state are to consist of 4 digits followed by 2 uppercase letters.
a) How many different license plates can there be in this state if repetition of letters and numbers is permitted?
b) How many different license plates can there be in this state if repetition of letters and numbers is not permitted?
c) How many different license plates can there be in this state if the must be , and repetition of letters and numbers is not permitted?
d) How many different license plates can there be in this state if the first digit cannot be , and repetition of letters and numbers is not permitted?