posted by marco on .
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
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.