Question

A bag contains 6 red balls and some blue balls. If the probability of drawing a blue ball from the bag is twice the probability of drawing a red ball, find the number of blue balls.

Answer 1

Let the number of blue balls in the bag be x.
Then, total number of balls = ( 6 + x)
Given: P(blue ball) = 2 ⨯ P( red ball)
∴ $\frac{x}{6+x}=2\times \left(\frac{6}{6+x}\right)\Rightarrow x=12$
Hence, the number of blue balls in the bag is 12.