How many four digit numbers are there? Assume that 0 can't be a first digit. I got 9,000. Is that right?
Follow up question: How many of those four digit numbers have no repeated digits?

Equivalent question:  You have a bag with 10 marbles in it labeled 0 through 9. You pick out a marble 4 times and place it back in the bag each time.  The first time, there is no marble with a 0.  The total combinations of picking and replacing 4 marbles with the no-zero-condition on the first pick is equal to the product of how many marbles are in the bag for each pick:
$$9 \times 10 \times 10 \times 10 = 9000.$$
For no repeated digits, it's the same problem, but you don't replace the marble (but on the second pick, you place the 0 marble back in):
$$9 \times 9 \times 8 \times 7 = 4536$$
