Find the total number of outcomes when choosing the last
three digits in a five-digit zip code if the first digit is 6 and the
second digit is 1, and no digit is used more than once.

There are 10 digits in total (0, 1, 2,...,9).
Two digits are in use already.
Since no digit is used more than once, we have the following:
For the 3rd digit, there are 10-2=8 choices.
For the 4th digit, there are 10-3=7 choices.
For the 5th digit, there are 10-4=6 choices.

Thus the total number of outcomes is 8*7*6=336

:)