Answer
676,000
Work Step by Step
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.
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Each of the first three digits can be chosen in only one way.
Ways of choosing the
1st letter: 26
2nd letter: 26 (can be the same as the first),
1st digit: 10 (any of 0-9),
2nd digit: 10,
3rd digit: 10.
Total: 26$\times$26$\times$10$\times$10$\times$10=676,000
