Question

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

• A. $5$
• B. $10$
• C. $2$
• D. $15$

Answer 1

The correct option is B $10$

$Let$ $the$ $total$ $number$ $of$ $blue$ $balls$ $be$ $x.$
$Total$ $balls$ $=5+x.$
$Thus$ $probability$ $of$ $taking$ $out$ $a$ $red$ $ball$ $=\frac{5}{5+x}$ $and$ $that$ $of$ $a$ $blue$ $ball$ $=\frac{x}{5+x}$
$Given$ $that$ $\frac{x}{5+x}=2\times \frac{5}{5+x}$
$\therefore$ $x=10$