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.

Explanation

Number of white balls = 15 Let x be the number of black balls. Total number of balls in the bag = 15 + x Also, the probability of drawing a black ball from the bag is thrice that of drawing a white ball. ⇒ x/(15 + x) = 3[15/(15 + x)] ⇒ x = 3 × 15 = 45 Hence, the number of black balls in the bag = 45