Current rules for telephone area codes allow the use of digits 2-9 for the first digit and 0-9 for the second and third digits. how many different area codes are possible with this rules? that same rule applies to the exchange number. which are the three digits immediately preceding the last four digits of a phone number. given both of those rules, how many 10- digit phone numbers are possible? given that rules apply to the united states and canada and a few islands, are there enough possible phone numbers? (assume that the combined population is about 400,000,000.