Two boxes contain 4 and 16 balls respectively Two balls in the first box and four in the second are ...........

Quantitative Aptitude (SBI-CL) Probability

Two boxes contain 4 and 16 balls respectively. Two balls in the first box and four in the second, are black. If a box is chosen randomly and two balls are drawn at random from it, what is the probability that at least one ball is black if the ball is not replaced?

11/20

43/120

77/120

9/20

None of these

Explanation:

At least one black can be chosen in three ways:
1. first one is black, second is non-black
2. first one is non-black, second is black
3. both are black.
Probability of selecting a box is 12
Now, probability of choosing at least one black ball