A bag contains 15 white and some black balls

A bag contains 15 white and some black balls. If the probability of drawing a black ball from the bag is thrice that of drawing a white ball, find the number of black balls in the bag.

Answer

Let the number of black balls in the bag be n.

Total number of balls are 15 + n

P(Black ball) = 3 × P(White ball)

$$ \frac{n}{15+n} = 3 \times \frac{15}{15+n} $$                                                                                                                                 

n = 45