Question

A bag contains $5$ red balls and some blue balls. If the probability of drawing a blue ball is double that of a red ball, then the number of blue balls in the bag are:

• A. $20$
• B. $15$
• C. $12$
• D. $10$

Answer 1

The correct option is D $10$

Number of red balls $=5$

Let number of blue balls $=x$

Total number of balls $=5+x$

Probability of drawing one blue ball $=\frac{x}{x+5}$

Probability of red ball $=\frac{5}{x+5}$

Probability of drawing one blue ball $=2x$ Probability of red ball

$\Rightarrow \frac{x}{x+5}=2\times \frac{5}{x+5}$

$\Rightarrow x=10$