The keyless entry system in some cars uses a 4-digit keypad. there are 10 possible digits, and the digits can be repeated. how many different combinations are there?

This item can be answered through the concept of fundamental principles of counting. In the first of the four digits, there are 10 possible digits. The same with all the other 3 places or digits of the code. That is,
                             n = 10 x 10 x 10 x 10 
Giving us the answer of 10,000. Thus, there are 10,000 possible keys for the keyless entry.