Question

A bag contains 5 red balls and some blue balls.If the probability of drawing a blue ball from the bag is four times that of red ball,find the number of blue balls in the bag.

Answer 1

Let the number of blue balls be $x$

So total number of balls $=5+x$

P(probability of drawing a red ball from the bag)$=\frac{5}{5+x}$

P(probability of drawing a blue ball from the bag)$=4\frac{5}{5+x}$

ATQ,

$\frac{x}{5+x}=4\frac{5}{5+x}$

$\frac{x}{5+x}=\frac{20}{5+x}$

$5x+x^2=100+20x$

$x^2-15x+-100=0$

$(x-20)(x+5)=0$

So $x\ne -5$

Therefore No. Of blue balls $=20$