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 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.