Probability

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tsmith93: Senior | Next Rank: 100 Posts; Posts: 32; Joined: Fri Feb 19, 2010 12:36 pm; Location: Pennsylvania; Thanked: 1 times

by tsmith93 » Fri Mar 26, 2010 5:35 am

A jar has 5 marbles, 1 of each of the colors red, white, blue, and yellow. If 4 marbles are removed from the jar, what is the probability that the yellow marble was removed?

(a) 1/20
(b) 1/5
(c) 1/4
(d) 4/5
(e) 5/4

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Source: Beat The GMAT — Problem Solving | Fri Mar 26, 2010 5:35 am

eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 amFollowed by: 1 member; Thanked: 32 times

by eaakbari » Fri Mar 26, 2010 5:47 am

I think you forgot to list one more colour of marbles.
Nonetheless

Instead of finding probability that the yellow marble was removed lets find the probability that it was not removed

that is P(Yel left) = 4/5 *3/4 * 2/3 *1/2

P(yel gone) = 1 - p (yel left )

= 96 / 120 =4/5

Therefore OA = D

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thephoenix: Legendary Member; Posts: 1560; Joined: Tue Nov 17, 2009 2:38 amFollowed by: 5 members; Thanked: 137 times

by thephoenix » Fri Mar 26, 2010 8:44 am

tot # of ways for selecting 4 marbles out if 5 is 5c4=5 ways

# of selecting 4 marbles out of 4 ( leaving yellow marble) is 4c4=1 way
so P of not picking yellow marble=1/5
hence P of picking a yellow marble=1-1/5=4/5

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harshavardhanc: Legendary Member; Posts: 526; Joined: Sat Feb 21, 2009 11:47 pm; Location: India; Thanked: 68 times; GMAT Score:680

by harshavardhanc » Fri Mar 26, 2010 1:19 pm

tsmith93 wrote:A jar has 5 marbles, 1 of each of the colors red, white, blue, and yellow. If 4 marbles are removed from the jar, what is the probability that the yellow marble was removed?

(a) 1/20
(b) 1/5
(c) 1/4
(d) 4/5
(e) 5/4

there are 5C4 or 5 ways to select 4 colored balls out of 5.

out these 5 ways, each color will be present in 4 of the ways. hence, required prob = 4/5 .

Regards,
Harsha

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