Say we use four elements where three elements are same and one is different A,A,A,B now total number of possible combinations are A, B, AA, AB, AAA, AAB, AAAB, So ans=7
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1Doesn't $BA$ count as well? – Michael Hoppe Jun 07 '17 at 10:13
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The general method to solve problems like this is the method of generating functions. https://en.wikipedia.org/wiki/Generating_function#Ordinary_generating_function_.28OGF.29 – awkward Jun 07 '17 at 11:43
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No BA doesn't count we are taking combinations not permutations. – mukesh Jun 08 '17 at 07:59