SOLUTION: (a) How many seven-digit telephone numbers are possible if the first digit must be nonzero? 9000000 (b) How many international direct-dialing numbers for calls within the U

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Question 1206217: (a) How many seven-digit telephone numbers are possible if the first digit must be nonzero?

9000000
(b) How many international direct-dialing numbers for calls within the United States and Canada are possible if each 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)?
Answer by Edwin McCravy(20055) About Me  (Show Source):
You can put this solution on YOUR website!
I found this online:
Using 0 or 1 as the first digit of an area code or central office 
code is invalid, as is a 9 as the middle digit of an area code; 
these are trunk prefixes or reserved for North American Numbering 
Plan expansion.

Perhaps you should mention this to your teacher.

The trick for finding how many integers there are from A to B, inclusive,
is to subtract 1 less than the smallest from both the smallest and the largest:

You got the first correct, if you ignore the above.

100-0000 to 999-9999, or without the hyphen:
1000000 through 9999999

Subtract 1 less than the smallest, or 999999 from each

That's the integers 1 through 9000000.  You got that.

Doesn't the (b) part mean that you just put a 1 before each of those in the
(a) part?  Wouldn't it just be the same number?

Edwin