4. 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.
Total number of balls in the bag = 12
Number of black balls in the bag =
According to the question,
6 more black balls are added to the bag.
Total number of balls =
And, the new number of black balls =
The required value of is 3