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Each of the balls in a jar is either red, green or blue. If

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Manager
Joined: 10 Sep 2012
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Each of the balls in a jar is either red, green or blue. If [#permalink]

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30 Nov 2012, 20:21
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5% (low)

Question Stats:

85% (01:43) correct 15% (00:30) wrong based on 100 sessions

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Each of the balls in a jar is either red, green or blue. If one ball is removed at random, what is the probability that the ball will be green?

(1) There are 7 blue balls in the jar.
(2) The probability that the ball will be red is 1/4

[Reveal] Spoiler:
I thought this was C because I thought having number of blue balls would close the unknown down to exact numbers. Where am I mistaken? Is there a way to think through this in an elegant manner?
[Reveal] Spoiler: OA
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Re: Each of the balls in a jar is either red, green or blue. If [#permalink]

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30 Nov 2012, 20:35
1
KUDOS
anon1 wrote:
Each of the balls in a jar is either red, green or blue. If one ball is removed at random, what is the probability that the ball will be green?
(1) There are 7 blue balls in the jar.

(2) The probability that the ball will be red is 1/4

[Reveal] Spoiler:
I thought this was C because I thought having number of blue balls would close the unknown down to exact numbers. Where am I mistaken? Is there a way to think through this in an elegant manner?

Hii anon.
The probability that is given is just a kind of ratio in that $$R/(R+B+G)=1/4$$. If you combine this with # of B balls, then also there is not sufficient info to get the EXACT number of green balls.
There can be several numbers that can satisfy both the statements.
+1 E.
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Re: Each of the balls in a jar is either red, green or blue. If [#permalink]

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30 Nov 2012, 21:17
2
KUDOS
anon1 wrote:
Each of the balls in a jar is either red, green or blue. If one ball is removed at random, what is the probability that the ball will be green?
(1) There are 7 blue balls in the jar.

(2) The probability that the ball will be red is 1/4

[Reveal] Spoiler:
I thought this was C because I thought having number of blue balls would close the unknown down to exact numbers. Where am I mistaken? Is there a way to think through this in an elegant manner?

The easiest way to think through is always the most basic way. For knowing probability of anything we need to know favourable and total outcomes, in this case number of green balls and total balls. Or probability of it not happening ie not favorable/total in this case number of red+blue and total balls.

In the given question
Statement 1 doesnt help to get total or green ball or red+blue. Insufficient
Statement 2 gives only red/total. Insufficient
Combining these two we still can not find red+blue/total or green/total.

Therefore Ans E it is.
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Manager
Joined: 10 Sep 2012
Posts: 145
Re: Each of the balls in a jar is either red, green or blue. If [#permalink]

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02 Dec 2012, 08:59
Ah. yes. this was easy once I wrote out 1/4 and then 25/100, which gives an unlimited amount of numbers for which "red" could have been. Thanks guys!
Re: Each of the balls in a jar is either red, green or blue. If   [#permalink] 02 Dec 2012, 08:59
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