A seven-digit phone number in the united states consists of a three-digit exchange followed by a four-digit number.

a. the first digit of the exchange is not allowed to be a zero or a one. how many seven-digit numbers can be formed?

10 total numbers can be used for each digit (0-9) except the first digit:

1st digit can be 1 of 9 numbers

other 6 digits can be 1 of 10 numbers each

multiply all the possible numbers together:

9 x 10 x 10 x 10 x 10 x 10 x 10 = 9,000,000 possible phone numbers