5. A box contains 20 black and 30 green balls. One ball at a time is drawn at random, its color is noted, and the ball is then replaced in the box for the next draw.

(a) Find the probability that the first green ball is drawn on the fourth draw.

(b) Find the probability that the third and fourth green balls are drawn on the sixth and ninth draws, respectively.

(c) Let N be the trial at which the fifth green ball is drawn. Find the probability that the fifth green ball is drawn on the nth draw. (Note that N take values 5,6,7, . . . .)