Question

An urn contains $6$ blue and $a$ green balls. If the probability of drawing a green ball is double that of drawing a blue ball, then $a$ is equal to:

• A. $6$
• B. $18$
• C. $24$
• D. $12$

Answer 1

The correct option is D $12$

There are $6$ blue and $a$ green balls.

$⟹$ Total no. of balls $=6+a$

Now, $P$(blue ball) $=\frac{6}{a+6}$

and $P$(green ball) $=\frac{a}{a+6}$

$\because P\left(green\right)=2\times P\left(blue\right)$

$⟹\frac{a}{a+6}=2\times \frac{6}{a+6}$

$\therefore a=12$

Hence, option D is correct.