A box contains r red and b blue balls. One ball is selected at random and its color observed. The ball is then returned to the box and k additional balls of the same color are also put into the box. A second ball is then selected at random, its color is observed, and it is returned to the box togerther with k additional balls of the same color. Each time is another ball is selected, the process is repeated. If four balls are selected, what is the probability that the first three balls will be red and the fourth ball will be blue?

http://www.bilkent.edu.tr/~math250/Solution%20set2.pdf