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Explanation:
set A = dice values greater than 4 = {5,6}
set B = dice values 4 or less = {1,2,3,4}
Sets A and B are considered complementary because one or the other must happen. You must either roll something larger than 4 (set A) or you must roll something from set B.
There are 4 items in set B out of 6 possible outcomes. That leads us to the answer 4/6 = 2/3
