A box contains 12 balls out of which x are black. If one ball is drawn at random from the box, what is the probability that it will be a black ball? If 6 more black balls are put in the box the probability of drawing a black ball is now double of what it was before. Find x? (Ans: 3)
Ans: Number of all possible outcomes = 12
Number of outcomes favourable to the event of drawing black ball = x
Required probability = x/12
Now when 6 more black balls are put in the box, Number of all possible outcomes = 12 + 6 = 18
Number of outcomes favourable to the event of drawing a black ball = x + 6
∴ Probability of drawing a black ball = x + 6 /18
According to the question,
x + 6/18 = 2 ( x/12 )
∴ x = 3