A bag contains 15 balls of which some are white and others are red. If the probability of drawing a red ball is twice that of a white ball, find the number of white balls in the bag.

Let no. of white balls in bag = x, so no. of red balls = 15 - x.

P(drawing a white ball) = $\dfrac{\text{No. of favourable outcomes}}{\text{Total outcomes}} = \dfrac{x}{15}$

P(drawing a red ball) = $\dfrac{\text{No. of favourable outcomes}}{\text{Total outcomes}} = \dfrac{15 - x}{15}$.

Given, P(drawing a red ball) = 2 × P(drawing a white ball)

$\therefore \dfrac{15 - x}{15} = \dfrac{2x}{15} \\[1em] \Rightarrow 15 - x = 2x \\[1em] \Rightarrow 2x + x = 15 \\[1em] \Rightarrow 3x = 15 \\[1em] \Rightarrow x = 5.$

Hence, there are 5 white balls in the bag.