Probability

A bag contains some unmarked red balls, some red balls marked with '1', some unmarked blue balls and some blue balls marked with '1'. Red balls and blue balls are in the ratio 2 : 1. Probability of drawing red balls marked with '1' is r, probability of drawing blue balls marked with '1' is b. A ball is randomly selected from the bag,find the probability that the same marked ball is drawn in 2 consecutive draws in terms of r and b.