# A box contains some black balls and 30 white balls.If the probability of drawing a black ball is two fifths of white ball. Find the number of black ball

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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

total balls =30+x

P(black ball)=

5

A.T.Q.

x+30 5

5x = 2x+60

3x =60

x=20

therefore,the number of black ball is 20.