Question

How many possible phone numbers can we make out of XXX – XXXX, where X can be any number 0-9? with and without repetition.

Answer 1

Here is your 7 digit phone number represented with a blank line for each digit.
So the first x can be 10 different numbers _10__ ___ ___ ____ ____ ____ ____, but which ever number you use, it is then used up.
So then you only have 9 numbers left
_10_ x _9_ x ___ x ___ x ___ x ___ x ___
then once that number is used up you have one less number for the next digit of the phone number... and so on
10 x 9 x 8 x 7 x 6 x 5 x 4