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.

Let the no. of black balls in bag be x.
∴total no. of balls in bag = 15 + x
A ≡ event that black ball is drawn.
B ≡ event that white ball is drawn.
∴P(A) = x/(15+x) and P(B) = 15/(15+x)
P(A) = 3P(B) (given)
x/(15+x) = 3 × 15/(15+x)
∴x = 45
∴The no. of black balls in the bag is 45.