box contains 4 red and 7 green marbles If 9 marbles are draw

box contains 4 red and 7 green marbles. If 9 marbles are drawn without replacement, what is the expected number of red marbles? Answer:

Solution

Note that for a hypergeomtric distribution,

E(x) = K n / N

where

K = number of successes in the population = 4
N = population size = 4 + 7 = 11
n = sample size = 9

Hence,

E(x) = 4*9/11 = 3.272727273 [ANSWER]