Question

A bag contains 15 white and some black balls. If the probability of

drawing a black ball from the bag is thrice that of drawing a white ball,

find the number of black balls in the bag.

Answer 1

Here,
No. of white balls = 15
Let, the no. of black balls be 'x'
Then, total no. of balls = 15 + x
Now,
          P(drawing a white ball) = 15/(15 + x)
And,  P(drawing a black ball) = x/(15 + x)
But A/Q,
               x/(15 + x) = 3*15/(15 + x)
          => x  = 3*15
          => x  = 45
Therefore, there are 45 black balls in the bag.

Answer 2

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