Probability - black balls and white balls - The Beat The GMAT Forum

Probability - black balls and white balls

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Dec 21, 2008 10:54 am

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

Brent Hanneson - Creator of GMATPrepNow.com

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Source: Beat The GMAT — Data Sufficiency | Sun Dec 21, 2008 10:54 am

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Dec 21, 2008 11:06 am

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)

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Dec 21, 2008 11:13 am

Nice work, cramya! The answer is B.

I placed problem-solving version of this question on the PS side of the forum. [/quote]

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Dec 21, 2008 11:45 am

Brent ,thanks for continuing to post good questions!!!

Appreciate your contribution to this forum

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun Dec 21, 2008 11:50 am

Thanks cramya

Brent Hanneson - Creator of GMATPrepNow.com

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ronniecoleman: Legendary Member; Posts: 546; Joined: Sun Nov 16, 2008 11:00 pm; Location: New Delhi , India; Thanked: 13 times

by ronniecoleman » Mon Dec 22, 2008 12:59 am

IMO B

X/X+Y = 4/5

X = 4Y

X = Y+ 27

X = 36
Y = 9

...

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vittalgmat: Legendary Member; Posts: 621; Joined: Wed Apr 09, 2008 7:13 pmFollowed by: 4 members; Thanked: 33 times

by vittalgmat » Mon Dec 22, 2008 2:27 am

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

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