Question

A bag has white ,red, and black balls only . A ball is drawn at random from the bag . If the probability of getting a white ball is 3/18 & that of black ball is 2/5 then find the probability of getting red ball. If bag contains 20 black balls then find total number of balls in bag.

Answer 1

the probability of getting a white ball = 3/18
that of black ball = 2/5
the probability of getting red ball = 1- (3/18 +2/5)
= 1- 51/90 = 39/90 = 13/30

now second part ,
let the number of red balls be x and no of white balls be y
now probability of getting red ball = x/(x+y+20) = 13/30 -- eq i
probability of getting white ball = y/(x+y+20)= 3/18
---- eq ii
two equations and two variables
find x and y .................. from eq i --- 17x-13y =260
from eq ii ----------- 15y-3x= 60



total number of bags = (x+y+20)