Question

a bag contain 15 white and some black balls.If the probability of drawing a black ball is thrice that of drawing a white ball,find the number of black balls in the bag...

Answer 1

No.of black balls=15*3=45

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Answer 2

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.