drawing a white ball?

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Oct 22, 2011 4:16 am

In a jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?
(A) 1/5
(B) ¼
(C) 3/10
(D) 1/3
(E) ½

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Source: Beat The GMAT — Problem Solving | Sat Oct 22, 2011 4:16 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sat Oct 22, 2011 4:23 am

sanju09 wrote:In a jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?

The probability of drawing a red ball, returning it, and then drawing a white ball = (Probability of drawing a red ball)*(Probability of drawing a white ball)

Hence, 1/10 = (1/5)*(Probability of drawing a white ball)
Hence, Probability of drawing a white ball = (1/10)/(1/5) = 1/2

The correct answer is E.

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user123321: Master | Next Rank: 500 Posts; Posts: 385; Joined: Fri Sep 23, 2011 9:02 pmFollowed by: 6 members; Thanked: 62 times

by user123321 » Sat Oct 22, 2011 4:24 am

assuming x red and y green balls.
x/(x+y) = 1/5

(x/(x+y))*(y/(x+y)) = 1/10
substituting first equation
we get y/(x+y) = 1/2 which is nothing but prob. of drawing white ball

user123321

vaibhavgupta: Master | Next Rank: 500 Posts; Posts: 416; Joined: Tue Aug 30, 2011 2:18 pmFollowed by: 9 members; Location: Delhi, India; Thanked: 13 times

by vaibhavgupta » Sat Oct 22, 2011 4:26 am

sanju09 wrote:In a jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?
(A) 1/5
(B) ¼
(C) 3/10
(D) 1/3
(E) ½

IMO E thought they asked for green one
whats the OA?

sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Oct 22, 2011 4:43 am

Signify P as the probability of drawing a white ball from the jar. The probability of drawing a red ball is 1/5. The probability of drawing both measures is 1/10 so, 1/5 x P = 1/10.
Therefore P = [spoiler]½.

OA is E[/spoiler]

The mind is everything. What you think you become. –Lord Buddha

Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com

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