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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

Let number of black ball be x 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