An urn is filled with black balls and white balls only. If the probability of randomly drawing a white ball is 4/5, how many white balls must be added to the urn so that the probability of randomly drawing a white ball is 7/8?
(1) The ratio of white balls to black balls is 4:1
(2) There are 27 more white balls than black balls
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Probability - black balls and white balls
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Brent@GMATPrepNow
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cramya
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Prob is not my forte but anyways let me get better by giving this a shot.
Is it B)?
white balls = 36 black - 9 So 27 more white balls have to be added to make the ratio 7/8
Stmt I adds no new info. (4:1 part by part ratio can be obtained from the part to whole ratio 4/5 given in the question stem)
Is it B)?
white balls = 36 black - 9 So 27 more white balls have to be added to make the ratio 7/8
Stmt I adds no new info. (4:1 part by part ratio can be obtained from the part to whole ratio 4/5 given in the question stem)
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Brent@GMATPrepNow
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Nice work, cramya! The answer is B.
I placed problem-solving version of this question on the PS side of the forum. [/quote]
I placed problem-solving version of this question on the PS side of the forum. [/quote]
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B for me as well
We have W/(W+B) = 4/5
solving, W = 4B ------------- 1
Stmt 1:
Same as the q stem. Does not add anything more.
so N/S
Stmt 2:
W = B +27
substituting 1 above
4B = B +27
Solving B = 9.
So W = 36.
Now with this any new ratio can be created.
Hence B
We have W/(W+B) = 4/5
solving, W = 4B ------------- 1
Stmt 1:
Same as the q stem. Does not add anything more.
so N/S
Stmt 2:
W = B +27
substituting 1 above
4B = B +27
Solving B = 9.
So W = 36.
Now with this any new ratio can be created.
Hence B
