**Solution:** Total number of outcomes = 12

P(x) = x/12

When 6 more black balls are put in the box, total number of balls = 12 + 6 = 18

Number of black balls = 6 + x

P(black) = (x+6)/18

As per question;

(x+6)/18 = 2*x/12

or, x+6 = 3x

or, 2x = 6

or, x = 3