a. Consider assigning phone numbers to an area where each number consists of seven digits, each of which can be between 0 and 9. How many phone numbers can be assigned to the area?

b. To serve multiple areas, you decide to introduce an area code which serves as an identifier. In the U.S., area codes were initially three-digit numbers where the first digit ranged from 2 to 9, the second digit was either 0 or 1, and the third digit ranged from 1 to 9. How many areas could be specified using this system?

c. Using area codes with individual phone numbers, how many unique phones could this system support?