9. How many 10-letter words are there in which each of the letters e, n, r , s occur
(a) At most once? (b) At least once?
This is a binomial distribution question with
n = 10
p = 4/26 = 0.1538
q = 1 - p = 0.8462


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9. How many 10-letter words are there in which each of the letters e, n, r...
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