Math
posted by marco .
Confused on permutation or combination on this question:
A VISA credit card has a first digit of 4 followed by 15 other digits. How many different VISA accounts does this allow?
I know the formulas but I am having a hard time determining which one to use. Help!!!

We have a choice of 10 digits at each of the following digits, so the total number of choices is
10^(15).
In real life, the last digit (16th) is used for validation purposes, and is not a free choice. There number of "free" digits is therefore limited to 14. But that's not part of the math question. 
Thank you!