Need Help regarding Math Problem

Question

To use an ATM machine, you must enter a four digit code using the digits 0-9, How many four digit codes are possible if:

A) Allow repetition of digits:

B) Do not allow repetition of digits:

Answer 1

If repetition of digits is allowed: 10*10*10*10=10000

(there are a total of ten digits and 4 places to be filled)

If repetition of digits is not allowed: 10*9*8*7=5040