Question

A box $B_1$ contains $1$ white ball, $3$ red balls and $2$ black balls. Another box $B_2$ contains $2$ white balls, $3$ red balls and $4$ black balls. A third box $B_3$ contains $3$ white balls, $4$ red balls and $5$ black balls.

If 2 balls are drawn (without replacement) from a randomly selected box and one of the balls is white and the other ball is red, the probability that these 2 balls are drawn from box $B_2$ is

• A. $\frac{116}{181}$
• B. $\frac{126}{181}$
• C. $\frac{65}{181}$
• D. $\frac{55}{181}$

Answer 1

Answer: (D)

$\frac{55}{181}$

Let A: one ball is white and other is red

$E_1$ : both balls are from box $B_1$
$E_2$ : both balls are from box $B_2$
$E_3$ : both balls are from box $B_3$

Required Probability $=P\left(\frac{E_2}{A}\right)$
$=\frac{P\left(\frac{A}{E_2}\right)\cdot P\left(E_2\right)}{P\left(\frac{A}{E_1}\right)\cdot P\left(E_1\right)+P\left(\frac{A}{E_2}\right)\cdot P\left(E_2\right)+P\left(\frac{A}{E_3}\right)\cdot P\left(E_3\right)}$

$=\frac{\frac{{}^2{C}_1{\times }^3{C}_1}{{}^9{C}_2}\times \frac{1}{3}}{\frac{{}^1{C}_1{\times }^3{C}_1}{{}^6{C}_2}\times \frac{1}{3}+\frac{{}^2{C}_1{\times }^3{C}_1}{{}^9{C}_2}\times \frac{1}{3}+\frac{{}^3{C}_1{\times }^4{C}_1}{{}^{12}{C}_2}\times \frac{1}{3}}=\frac{\frac{1}{6}}{\frac{1}{5}+\frac{1}{6}+\frac{2}{11}}=\frac{55}{181}$