Answers

2014-02-25T12:31:51+05:30
Let p(b)=prob. of black ball
p(w)=prob. of white ball
total no of outcome=x+30
let x=no.of black ball
now
p(b)=2/5p(w)
so
x/30+x=30/(x+30)*2/5
so
x=2/5*30
x=12
so no. of black balls=12
2 3 2
2014-02-25T18:37:56+05:30
Let the black ball be x 
total balls =30+x
P(black ball)=2
                    5
A.T.Q.
  x      =  2
 x+30      5
5x = 2x+60
3x =60
x=20
therefore,the number of black ball is 20.

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