## Thinking Mathematically (6th Edition)

$10,000$
See page 690, The Fundamental Counting Principle: 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. --------- Each of the first three digits can be chosen in only one way. Ways of choosing the 4th digit ... 10 (any of the digits 0 - 9), 5th digit ... 10, 6th digit ... 10, 7th digit ... 10. Total codes possible (applying the Principle): 1$\times$1$\times$1$\times$10$\times$10$\times$10$\times$10$=10,000$ (This includes the number 699-0000. There are no constraints by which we should exclude it, so 10,000 remains as the answer)