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I am not sure which formula will allow me to get the following results.

If I have to pick two letters from A, B, C, D, then there are 6 possible combinations:

AB

AC

AD

BC

BD

CD.

Now assume that letter D has turned into another A, i.e. A_{2}. Now my combinations have changed to

A_{1}B

A_{1}C

A_{1}A_{2}

BC

BA_{2}

CA_{2}

So I eliminate BA_{2}, and CA_{2}and I have 4 unique combinations.

I know that in the case of permutations with repetitions, we divide the nPr formula by the factorials of the frequency of the letters, but I am not sure how it is done with combinations. What formula can use for combinations?

Thanks.

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# Formula for combinations with repeting objects

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