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.

### Answer

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.