Question

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

Answer 1

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