Two bags A and B contain 7 red and 6 blue balls respectively. Some blue balls from bag B are taken out and kept into bag A. If probability of selecting two blue ball from bag A is 1/15, find the number of blue balls drawn from bag B.

Let x be the number of blue balls moved from B to A. Bag A now has 7 red and x blue balls (total 7+x). Probability of 2 blue balls = [x/(7+x)] * [(x-1)/(6+x)] = 1/15. Solving x(x-1) / (x^2 + 13x + 42) = 1/15 gives 15x^2 - 15x = x^2 + 13x + 42, so 14x^2 - 28x - 42 = 0, or x^2 - 2x - 3 = 0. Roots are x=3 and x=-1. Thus, x=3.