Assume the following rules are true for any telephone number in North America:

a$.$ A telephone number in North America consists of ten numbers

o a three$-$digit area code,

o followed by a three$-$digit exchange$/$prefix$,$

o followed by a four$-$digit line$/$subscriber number

$($AAA$-$EEE$-$XXXX$).$

b$.$ The area code cannot start with a $0$ or $1.$

c$.$ The exchange cannot start with a $0$ or $1.$

d$.$ Other than the rules in b$.$ and c$.$ above, any digit $0-9$ can be used in any of the ten places in a phone number.