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Answer:
Step-by-step 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.