Telephone number in the unites states and Canada have 10 digits as follows ...

  

Telephone number in the unites states and Canada have 10 digits as follows :

i, 3-digit area code number, whose first digit cannot be 0 or 1

ii, 3-digit exchange number, whose first two digits cannot be 0 or 1.

iii 4-digit line number, whose digits cannot be all 0

a, how many area codes are possible?

b, how many 7-digit phone number are possible within the 617 area code

c, how many phone numbers are possible in the united states and Canada