Question

A bag contains 5 red and some blue balls. If the probability of drawing a blue ball is three times the probability of drawing a red ball then the number of blue balls in the bag is ............

• A. 10
• B. 12
• C. 15
• D. 8

Answer 1

The correct option is C 15

let x be the number of blue balls
Total balls $=5+x$
$P\left(red\right)=\frac{5}{5+x}$
$P\left(blue\right)=\frac{x}{5+x}$
$P\left(blue\right)=3\times P\left(red\right)$
$\Rightarrow \frac{x}{5+x}=\frac{3\times 5}{5+x}$
$\Rightarrow x=15$