I have tried to find a solution for this problem a lot but I couldn't solve it.
Consider a box containing 3 red ball and 7 white balls. Suppose that balls are drawn one at a time, at random, without replacement from this box until two red balls are obtained. Let X denote the number of the draw on which the second red ball is obtained. Find the probability mass function of X
all what I could do is to get the range for X. where X = 2 , 3, 4,5,6,7,8,9