Question

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

Answer 1

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