In a certain class, one student is to be selected at random : GMAT Data Sufficiency (DS)
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# In a certain class, one student is to be selected at random

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Senior Manager
Joined: 29 Jan 2011
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In a certain class, one student is to be selected at random [#permalink]

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31 May 2011, 15:38
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25% (medium)

Question Stats:

87% (01:20) correct 13% (00:29) wrong based on 31 sessions

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In a certain class, one student is to be selected at random to read. what's the probability that a boy will read.

(1) Two-thirds or the students in the class are boys
(2) Ten of the students in the class are girls

In second case we know the number of girls are 10 . So can we not find out the number of boys ? which will be 2/3 *3/1 * 10 = 20 . SO total class is of size 30 and probability that a boy will read is 20/30 which is 2/3 <--- probability ?

So shouldn't D be correct since we can find out probabilities from 1 and 2 both individually ?? Please explain
[Reveal] Spoiler: OA

Last edited by siddhans on 31 May 2011, 16:38, edited 1 time in total.
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31 May 2011, 15:56
siddhans wrote:
In a certain class, one student is to be selected at random to read. what's the probability that a boy will read.

(1) Two-thirds or the students in the class are boys
(2) Ten of the students in the class are girls

In second case we know the number of girls are 10 . So can we not find out the number of boys ? which will be 2/3 * 10 = 20 . SO total class is of size 30 and probability that a boy will read is 20/30 which is 2/3 <--- probability ?

So shouldn't D be correct since we can find out probabilities from 1 and 2 both individually ?? Please explain

When using statement 2, please ignore everything from statement 1. Statement 2 should prove its sufficiency independent of statement 1 to be considered independently sufficient.

For D to be correct, each of the statements should prove its sufficiency independently.

You used information from the first to find the number of students.
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31 May 2011, 16:36
fluke wrote:
siddhans wrote:
In a certain class, one student is to be selected at random to read. what's the probability that a boy will read.

(1) Two-thirds or the students in the class are boys
(2) Ten of the students in the class are girls

In second case we know the number of girls are 10 . So can we not find out the number of boys ? which will be 2/3 * 10 = 20 . SO total class is of size 30 and probability that a boy will read is 20/30 which is 2/3 <--- probability ?

So shouldn't D be correct since we can find out probabilities from 1 and 2 both individually ?? Please explain

When using statement 2, please ignore everything from statement 1. Statement 2 should prove its sufficiency independent of statement 1 to be considered independently sufficient.

For D to be correct, each of the statements should prove its sufficiency independently.

You used information from the first to find the number of students.

I see so when will C ever be an answer choice? When can we say that both the choices work together to get to an answer ?
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31 May 2011, 16:52
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siddhans wrote:
I see so when will C ever be an answer choice? When can we say that both the choices work together to get to an answer ?

When both the statements independently are NOT SUFFICIENT to answer the question asked but together are sufficient.

St1: Alone not sufficient.
St2: Alone not sufficient.
St1 and St2 Together: Sufficient.

Ans: "C"
***********************

Please look at the data sufficiency problems and work few examples to really absorb the concept.
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Re: Probaility   [#permalink] 31 May 2011, 16:52
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