Question

How many 7 digit phone numbers are there assuming the first digit can't be a 0 or a 1?

Answer 1

The first digit can be any one of the numbers 2-9 That a total of 8 numbers.

The next 6 digits can be any permutation of 6  from the numbers  0 to 9.
(10 numbers)

nPr  =  n! / r!

so here we have  10P6  = 10! / 6!  =  10*9*8*7  =  5040

So final answer  is 8 * 5040 = 40,320