A box contains 12 balls of which some are red in colour. If 6 more red balls are put in the box and a ball is drawn at random, the probability of drawing a red ball doubles than what it was before. Find the number of red balls in the bag.
Let number of red balls be = x
P(red ball) = x/12
If 6 more red balls are added:
The number of red balls = x + 6
P(red ball) = (x+6)/18
Since, (x+6)/18 = 2(x/12)
x = 3
Therefore, there are 3 red balls in the bag.