2002 · Q55
Bag 'A' contains 5 white and 2 black balls. Bag 'B' contains 2 white and 3 black balls. If any one bag is chosen and a ball is taken out of it at random, what is the probability that the ball is black?
General Mental Ability, Quantitative Aptitude and StatisticsNumber system and arithmetic
- A31/70
- B1/2
- C5/12
- D3/5
Explanation
Choosing a bag at random (probability ½ each): P(black) = ½×(2/7) + ½×(3/5) = 1/7 + 3/10 = 10/70 + 21/70 = 31/70. Note: the printed guide-book key marks (B), but its working wrongly uses 2/5 for bag A — bag A contains 5 white + 2 black = 7 balls, so the correct fraction is 2/7. Hence (A) is correct.