Question

A bag contains 40 balls out of which some are red, some are blue and remaining are black. If the probability of drawing a red ball is $\frac{11}{20}$ and that of blue ball is $\frac{1}{5}$, then the number of black ball is?

• A. $5$
• B. $25$
• C. $10$
• D. $30$

Answer 1

Answer: (C)

$10$

Let no. of red balls = x, no. of blue balls =y and no.of black balls = z.
P (red ball) =$\frac{11}{20}=\frac{x}{40}$
$\Rightarrow$ x=22
P (Blue ball) = $\frac{1}{5}=\frac{y}{40}$
$\Rightarrow$ y=8
$\therefore$ No of black balls = $40-22-8=10$