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 = 12
Total number of black balls = x
P (getting a black ball) = 
If 6 more black balls are put in the box, then
Total number of balls = 12 + 6 = 18
Total number of black balls = x + 6
P (getting a black ball now) = 
According to the given condition,
2 (
) = 
3x = x + 6
2x = 6
x = 3
5