Question

A bag contains white,black and red balls only. A ball is thrown at random from the bag. If the probability of getting a white ball is 3/10 and of a black ball is 2/5 then find probability of getting a red ball. If the bag contains 20 black balls,the find the total number of balls in the bag

Answer 1

Solution :

Let the no of white balls be = x

Let the no of red balls be = y

number of balls black balls =20

total number of balls =x+y+20=x+y+20

Probability of getting a white ball =310=310

xx+y+20=310xx+y+20=310

=> 10x=3x+3y+6010x=3x+3y+60

=> 7x−3y=607x−3y=60------(1)

Probability of getting a blind ball =25=25

(i.e)20x+y+20=2520x+y+20=25

= 100=2x+2y+40100=2x+2y+40

=> 2x+2y=602x+2y=60

or x=y=30x=y=30 ----------(2)

Solving (1) and (2) we get,

7x−3y=607x−3y=60

x+y=30x+y=30

______________

7x−3y=607x−3y=60

7x+7y=2107x+7y=210

________________

−10y=150−10y=150

=> y=15y=15

x=15x=15

Answer : 15
i hope this will help u