A bag contains 15 white and some black balls

• Probability (C10)

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.

Solution

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)

n/(15+n) = 3 × 15/(15+n)

n = 45