Combinatoricshard
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How many subsets of {1, 2, 3, ..., 20} have the property that the sum of elements is divisible by 4?
How many subsets of {1, 2, 3, ..., 20} have the property that the sum of elements is divisible by 4?
How many subsets of {1, 2, 3, ..., 20} have the property that the sum of elements is divisible by 4?