I was wondering how can we can count how many available combinations in simple school locker ? And then if we can how many possible number of credit cards can be issued ?
For each piece you have 10 possibilities, the numbers 0,1,2,3,4,5,6,7,8,9.
You have four of those numbers. Hence there are 10 x 10 x 10 x 10 = 10^4 = 10.000 possibilities.
Can you now try to answer your own question for the creditcard?