A box contains 5 red and 7 green marbles If 6 marbles are dr

A box contains 5 red and 7 green marbles. If 6 marbles are drawn without replacement, what is the expected number of red marbles?

Solution

Note that

E(x) = K n / N

where

K = number of successes in the population = 5
N = population size = 5 + 7 = 12
n = sample size = 6

Thus,

E(x) = 5*6/12 = 2.5 [ANSWER]