## Thinking Mathematically (6th Edition)

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. ------------ 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