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Question

A box contains 12balls out of whichxare black. If one ball is drawn at random from the box, what is the probability that it will be a black ball? If 6more black balls are put in the box, the probability of drawing a black ball is now double of what it was before. Findx.


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Solution

Given that, number of black balls =x

and total number of balls=12

So, the required probability is p(x)=x/12

When 6 more black balls are put in the box, total number of balls =12+6=18 then

Number of black balls =6+x

And probability of black ball is p(black)=x+618

as per condition

x+618=2x12
x+6=3x6=3x-x2x=6x=3
Hence, number of black balls=3


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