I'm taking a Mathematical Statistics class, and the first chapter is on probability. Most of it makes sense, but I'm still trying to understand how to tackle certain problems (when to use permutations over combinations, etc.)

Here is one problem (out of several) that I'm having trouble trying to set up:

Source: *Probability and Statistical Inference*, 7e by Hogg and Tanis. Problem 1.3-7

I'm pretty sure that the number of possible 4-digit combinations is , but I'm not sure where to go from there.

I'd appreciate any help!

--Chris