(a) How many different 7-digit telephone numbers are possibl…

(a) How many different 7-digit telephone numbers are possible if repetitions are allowed?   (b) How many different 7-digit telephone numbers are possible if the first digit cannot be 0 or 1, and repetitions are allowed?   (c) How many different 7-digit telephone numbers are possible if the first digit must be a 3 and repetitions are not allowed?