Question

A bag contains $20$ balls out of which $x$ are black. If $10$ more black balls are put in the box, the probability of drawing a black ball is double of what it was before. The value of $x$ is

• A. $0$
• B. $5$
• C. $10$
• D. $40$

Answer 1

The correct option is B $5$

Probability of black balls $=\frac{x}{20}$
Probability of black balls (New)$=\frac{x+10}{30}$
$\frac{x+10}{30}=\frac{2x}{20}$ (Given)
$\Rightarrow x=5$