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I have N couples of real numbers higher than 1.

Let's call them like (a0,b0), (a1,b1),...,(aN,bN)

I have a number R <= N.

I need the sum of all the possible products of N elements, chosing one from each couple but exactly R times the "b" element and N-R times the "a" element.

Which is the best way to do it?

As an example:

(2,3), (5,7), (11,13)

N = 3, R = 2

I need 2x7x13 + 3x5x13 + 3x7x11

Thank you!

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# Sum of all possible products of elements taken from couples

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