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Here's the Solution to this Question
For this questions, there are two situations
When repetition is not allowed
There are 9 numbers and we want to form 3 digit numbers. Three digit numbers that can be formed is, numbers.
Therefore when repetition is not allowed, we can form 504 three digit numbers.
When repetition is allowed.
When we are allowed to repeat digits, then there are three digit numbers that can be formed.
Therefore, when repetition is allowed, we can form 729 three digit numbers.