13. How many five-digit numbers can be formed from the set of digits {1, 2, 3, 4, 5, 6, 7, 8, 9}, if no digit can appear more than twice? (For example, 77287 is not allowed but 77289 is.)
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If no digit is repeated, then we have 9 choices for the first digit, 8 choices for the second digit, 7 choices for the third digit, 6 choices for the fourth digit, and 5 choices for the fifth digit. So there are $9 \times 8 \times 7 \times 6 \times 5$ such Show more…
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