申論題

5. In a box there are R red balls and B black balls. In an experiment, we randomly pick a ball from the box, record its color and then throw the ball back to the box. We then repeat the random picking N times. Let X be the total number of red balls picked. We assume R, B and N are all positive integers.

(2)  We now change the experiment setup. When a ball is picked, it is removed from the box for the rest of the experiment. Let N be no greater than R+B. What is the mean value of X now?