How many different sequences are the in a 7 digit number?

Answer 1

10 digits to select for the first number in the sequence

10 digits to select for the second number in the sequence for each possible option of 10 digits in the first slot in the sequence (10 * 10)

10 digits to select for the third number in the sequence for each possible option of 10 digits in the second slot for each possible digit in the first slot of the sequence (or more easily put, 10 possible digits for each of 100 potential first two number sequences) [10*10*10]

etc....

=10 * 10 * 10 * 10 * 10 * 10 * 10 = 10^7 = 10,000,000 (10 Million)

Now add an area code before that 7 digit phone number and you get 10 Billion combinations (of course there are exclusions such as any sequence starting in 911 would be prohibited which is a reduction of 10 million numbers itself - 911 000 0001, 911 000 0002, etc. There are many other exclusions as well such as can't start with 0 - this removes another 1 billion sequences)