Question

You have been assigned a nine-digit identification number.

Should you use the fundamental counting principle or a tree diagram to find the total number of possible identification numbers?​

Answer 1

Explanation:

A tree diagram? I don't think so. Probably it would take a year's supply of paper to record all the possibilities. The question really is do you allow duplications and is 0 allowed to be the first digit.

I would think that both are fine.

So the total number for this number would be 10^9. There probably some sort of algorithm to reduce the number and to check if it is a legitimate choice for this number.