Question

A bag contains 15 white and some black balls.If the probability of drawing a black ball from the bag is thrice that of drawing a white ball find the number of black balls in the bag

Answer 1

No. of white balls=15

let the no. of black balls = x

probability of drawing a white ball = 15/(15+x)

probability of drawing a black ball = x/(15+x)

According to given condition-

x/(15+x)=3*15/(15+x)

x=3*15

 =45

therefore the no. of black balls = 45

Answer 2

Number of white balls = 15

Let total number of balls be x.

So, Number of black balls = x - 15.

Probability of drawing a white ball, P ( W) = 15 /x

Probability of drawing a black Ball, P( B)
= (x - 15 ) /x

ATQ,

P( B) = 3 P ( W )

( x - 15) /x = 3 × 15 / x

x - 15 = 45

x = 45 +15 = 60.

Number of black balls = ( x - 15 ) = 60 - 15 = 45