Sum[Binomial[40, n]*(1/1000)^(n−1)*(999!/((999−40+n)!*1000^(999−40+n))), {n, 2, 39}]Sum[Binomial[40, n]*(1/1000)^(n−1)*(999!/((999−40+n)!*1000^(999−40+n))), {n, 2, 39}]

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