Question

A word processing program requires a user to enter a 7-digit registration code made up of the digits 1,2,4,5,6,7 and 9. Each number has to be used, and no number can be used more than once. How many codes are possible?

Answer 1

For the first digit there are 7 choices. For the second digit there are 6 choices (because we can't use the same one again). And so on, until there is only one choice for the last digit.

The number of possible codes is this:
7*6*5*4*3*2*1 = 7! = 5040