# 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

1
Log in to add a comment

Log in to add a comment

now total ball will be (30 + x)

probability of getting black ball, P(B) = x/(30+x)

and , probability of getting white ball , P(W) = 30/(30+x)

now, A/Q,

x/(30+x) =2/5 ×(30/(30+x)

⇒x = 12

so, number of black ball is 12