#### Answer

4,500

#### Work Step by Step

The number of ways in which a series of successive things can occur is found by multiplying the number of ways in which each thing can occur.
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We want to see in how many ways we can choose
the 1st digit, .... ( 9 ways, one of 1-9, can not be 0)
the 2nd digit, .... ( 10 ways, one of 0-9)
the 3rd digit, .... ( 10 ways, one of 0-9)
the 4th digit, .... ( 5 ways, one of $\{1,3,5,7,9\}$, an odd digit)
Since this is a series of successive choices,
we apply the FTC.
Total= 9$\times$10$\times$10$\times$5=4,500