MATH HELP its driving me nuts

Question

Until​ recently, all area codes had a 0 or a 1 as the middle digit and the first digit could not be 0 or 1.How many area codes were possible with this​ arrangement? How many telephone numbers does the current 9​-digit sequence permit per area​ code? (The first three digits that follow the area code cannot start with 0 or 1 or 2. Assume that there are no other​ restrictions.)

Answer 1

It's simpler than you think.

The first digit has only 8 possibilities.

The third digit (middle number) has only 2 possibilities.

The second digit has 10 possibilities.

The total number of combinations is (8)(10)(2)=160

With normal numbers, each digit can be any of 10 digits possible combinations is 10^N where N is the number of digits.

Possible 9-digit numbers:per area code:We're only talking about the last 6 digits, the first of which is restricted to 7 possibilities. The other 5 can use all 10 digits.

n = (10⁵)(7) = 700,000.