An urn contains 10 white and 5 red marbles. Three marbles are selected at random without replacement from the urn. Set up the probability distribution for the randomn variable X defined as the number of red marbles selected. Determine the expected value and the variance of X.
So what I did
P(WWW) = 0.2637
P(WWR) = 0.1648
P(WRW) = 0.1648
P(WRR) = 0.0733
P(RWW) = 0.1648
P(RWR) = 0.0733
P(RRW) = 0.0733
P(RRR) = 0.0220
The mean is 1 and the variance is 0.5056.
My problem is, I keep on getting 0.5714, where am I going wrong?
So what I did
P(WWW) = 0.2637
P(WWR) = 0.1648
P(WRW) = 0.1648
P(WRR) = 0.0733
P(RWW) = 0.1648
P(RWR) = 0.0733
P(RRW) = 0.0733
P(RRR) = 0.0220
The mean is 1 and the variance is 0.5056.
My problem is, I keep on getting 0.5714, where am I going wrong?