5) Telephone numbers in the

United States begin with a

3 digit area code followed

by 7 digits. Area codes and

local telephone numbers

cannot begin with O or I.

How many telephone

numbers are possible?

6,400,000,000 (6.4 billions)

Step-by-step explanation:

We have a total of 10 digits, and each digit was 10 possibilities of being filled, except the first digit of the area code and first digit of the telephone number, that have only 8 possibilities.

So if 8 digits have 10 possible values and 2 digit has 8 possible values, the number of possible telephone numbers is:

10^8 * 8^2 = 6,400,000,000 (6.4 billions)