Explanation:
For the first slot, we have 9 digits to choose from (1 through 9).
Whatever we pick for the first choice, we have 9 choices left for the second slot. Say we picked 2 for the first slot. That means we have {0,1,X,3,4,5,6,7,8,9} to choose from. I used X to indicate we can't pick that digit any more. You'll see there are 9 items in that list.
Then for the third slot, we had 8 choices to pick from
Overall we have 9*9*8 = 81*8 = 648 different area codes possible where the first digit cannot be zero.
