Question 10
For years, telephone area codes in the United States and Canada consisted of a sequence of three digits. The first digit was an integer between 2 and 9, the second digit was either 0 or 1, and the third digit was any integer from 1 to 9.
a) How many area codes were possible?
b) How many area codes starting with a 4 were possible?
**Editorial note for this self-study exercise:** Use exactly the digit rules stated here, with both endpoints included; this is a mathematical counting model, not a claim about present-day telephone numbering. Enter the answers to (a) and (b), in that order, separated by a semicolon with no spaces.
a) How many area codes were possible?
b) How many area codes starting with a 4 were possible?
**Editorial note for this self-study exercise:** Use exactly the digit rules stated here, with both endpoints included; this is a mathematical counting model, not a claim about present-day telephone numbering. Enter the answers to (a) and (b), in that order, separated by a semicolon with no spaces.