3 different cases formed Case I : When both red balls are chosen, then both balls are removed from the bag. Case II : When both black balls are chosen, then both balls are again removed from the bag. Case III : When two different balls are chosen, the red ball is returned to the bag. The bags in the balls after second last trial will be (red, black) or (red, red, red). ∴ In both cases, there would be a red ball in the bag. Hence, the probability of red ball in bag is 1.