Suppose that X is a negative binomial random variable with p = 0.2 and k = 4. Determine the following:
a. P(x=19)
b. P(x=20)
c. P(x=21)
a) 18nCr3 · (0.2)^4 · (0.8)^(19 - 4) = 0.046
b) 19nCr3 · (0.2)^4 · (0.8)^(20 - 4) = 0.044
c) 20nCr3 · (0.2)^4 · (0.8)^(21 - 4) = 0.041
+82 
