Dog gone probability. Will choose best answer?

1. First digit has 9 possibilities and the other six have 10 possibilities each

hence 9x10^6.

2. Same as above for 3 digit numbers : total = 900

put this together :900 x 9x10^6. = 81x10^8.

How many seven-digit telephone numbers are possible if the first digit must be nonzero?

= 9 X 10^6

ach number consists of a 1 plus a three-digit area code (the first digit of which must be nonzero) and a number of the type described in part (a)?

= 1 X (9 X 10^2) X (9 X 10^6 )

= 81 X 10^8

QED

9*10*10*10*10*10*10

9 choices for the first digit, 10 choices for the rest

9,000,000

b) 1*(9*10*10)*9,000,000

First number must be 1, 1 choice

Area code is 3 digits, the first cant be zero

9*10*10

900 choiced

the rest of it is part a

so...

1 * 900 * 9,000,000

= 8,100,000,000

Is there a separate section for voting? I don't understand how you vote in the first place. Since most of you seem to know how this forum works, why do all the top contributors have their questions and answers private? Sometimes, when someone answers a question well I want to look at their other answers and gain knowledge. There is one poster I think it is something like ND DVM which I believe he is probably a vet in reading his responses. I looked at his answers that were not blocked in privacy and don't understand why people block theirs. Is there a reason for blocking this information?