a 7 digit phone number has a 3 digit prefix followed by 4 digits.how many different phone numbers are possible??

Answer 1

I assume 0 through 9 are allowed and repeats are possible. For example 000-0000 is allowed.

How many prefixes?
10*10*10 = 1,000
how many digits
10*10*10*10 = 10,000
so 10^3 * 10^4 = 10^7 = 10 million
I guess that is why they needed area codes.

Answer 2

7!=1*2*3*4*5*6*7=... (combinatoric)