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.
Anna only has four kittens and two are black which makes it half of her litter. So we would do the same with Kevin. Since he has ten kittens half of them would be black.