Acode has 6 digits. you can type in 0~9 for your code. how many different combinations of codes can you make if you can repeat digits? how many different combinations of codes can you make if you can not repeat digits?

Suppose you are choosing a 6-digit personal access code. this code is made up of 4 digits chosen from 1 to 9, followed by 2 letters chosen from a to z. any of these digits or letters can be repeated. find the total number of personal access codes that can be formed. 492,804 341,172 39,917,124 4,435,236