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If given ABBC, find the permutations if 2 letters are picked. If I calculate manually, they are: AB, AC, BA, BB, BC, CA, CB, 7 permutations. Is there any formula to solve this type of question? (Meaning, picking r number of letters from n number of letters with duplicated letters)

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    For this type of question, I am meaning, picking r number letters from n number letters with k duplicated.2011-08-15

2 Answers 2

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Suppose you had $n$ unique letters ${X_k}$, and letter $X_k$ were present in $r_k$ copies.

Then the number of unique pairs of letters, can be computed as $n (n-1) + \sum_{k} \mathrm{sgn} (r_k-1)$.

The term $n(n-1)$ count the number of pairs where letters are distinct, and remaining sum counts same letter pairs.

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    @Sasha Thank you for your explanation.2011-08-15
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Permutation of n things taken r at a time out of which k are repeated is

$\frac{n!}{(n-r)!k!}$

In your question , n=4 r=2, k=2

so total permutations is $\frac{4!}{2!2!}$=6

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    Allen has shown you 7 permutations, so if your formula gives 6, then something is wrong with your formula, or something is wrong with mathematics.2011-08-15