Suppose a set S has 11 elements. How many subsets of S have an even
number of elements? Express your answer as a summation.

I'm not really sure how to approach this. I think the general formula for this type of question is n!/k!(n-k)! where n is 11 and k is the number we're choosing. So would we just take the summation starting at 0 to 5 of 11!/2i!(n-2i)! Or what?