In decimal, 4 digits ( 0  9) you can represent any number from 0000 to 9999. That is 10,000 unique number combinations. Incidentally, the number of unique cominations is 10 to the
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You have 10 choices for the first digit, then 10 choices
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You have 5 choices for the first number, 5 choices for the second,
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http://www.mathwords.com/p/permutation_f…. nPk is n numbers and you order k of them. in specific orders. 3P3 = 3!/(3 3) = 3! = 6. if you add a single digit as duplicate .
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