There are 6×6×6 = 216 possible outcomes. Of those, 5×5×5 = 125 of them will be outcomes that do not have any occurrences of 6. The number that will have at least one occurrence of 6 is 216 - 125 = 91.
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5×5×5 counts the number of ways that the numbers 1–5 might be rolled on 3 rolls.