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I have a question and would be pleased if you help me by suggesting a paper or book to study.

Let A={1,2,....,n}. We consider all the subsets with k elements. How many of these sets have a sum of r ?

e.g.

for n=6, k=3, r=10

{1,3,6}

{1,4,5}

{2,3,5}

Hense the solution in this case is 3.

Thanks for your kind.