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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In a certain class, one student is to be selected at random [#permalink]

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20 May 2010, 09:15
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In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?

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

OPEN DISCUSSION OF THIS QUESTION IS HERE: in-a-certain-class-one-student-is-to-be-selected-at-random-136197.html
[Reveal] Spoiler: OA
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Re: In a certain class, one student is to be selected at random [#permalink]

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20 May 2010, 09:26
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targetMBA11 wrote:
In a certain class, one student is to be selected at
random to read. What is the probability that a boy
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.

A,

St 1, -- 2/3 students are boys, hence probability of selecting a boy is 2/3. Hence Sufficient.

St 2, --It only mentions the number of girls in the class but we dont know the total number of students. Say for e.g there are total 30 students then the probability of selecting a boy will 2/3 but if there are 40 students then the probability of selecting a boy will be 3/4 . Hence not Sufficient.
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Re: In a certain class, one student is to be selected at random [#permalink]

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20 May 2010, 09:32
How do you say that probability will be 2/3 only i didnt understand this part. Until u dont know the complete no of students how can we find one ?

nitishmahajan wrote:
targetMBA11 wrote:
In a certain class, one student is to be selected at
random to read. What is the probability that a boy
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.

A,

St 1, -- 2/3 students are boys, hence probability of selecting a boy is 2/3. Hence Sufficient.

St 2, --It only mentions the number of girls in the class but we dont know the total number of students. Say for e.g there are total 30 students then the probability of selecting a boy will 2/3 but if there are 40 students then the probability of selecting a boy will be 3/4 . Hence not Sufficient.
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Re: In a certain class, one student is to be selected at random [#permalink]

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20 May 2010, 09:34
3
KUDOS
targetMBA11 wrote:
How do you say that probability will be 2/3 only i didnt understand this part. Until u dont know the complete no of students how can we find one ?

nitishmahajan wrote:
targetMBA11 wrote:
In a certain class, one student is to be selected at
random to read. What is the probability that a boy
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.

A,

St 1, -- 2/3 students are boys, hence probability of selecting a boy is 2/3. Hence Sufficient.

St 2, --It only mentions the number of girls in the class but we dont know the total number of students. Say for e.g there are total 30 students then the probability of selecting a boy will 2/3 but if there are 40 students then the probability of selecting a boy will be 3/4 . Hence not Sufficient.

Suppose total students = x
number of boys = 2/3 * x

probability = total boys/total students = (2x/3 ) / x = 2/3
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Senior Manager
Joined: 25 Jun 2009
Posts: 306
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Kudos [?]: 122 [1] , given: 6

Re: In a certain class, one student is to be selected at random [#permalink]

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20 May 2010, 09:37
1
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targetMBA11 wrote:
How do you say that probability will be 2/3 only i didnt understand this part. Until u dont know the complete no of students how can we find one ?

nitishmahajan wrote:
targetMBA11 wrote:
In a certain class, one student is to be selected at
random to read. What is the probability that a boy
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.

A,

St 1, -- 2/3 students are boys, hence probability of selecting a boy is 2/3. Hence Sufficient.

St 2, --It only mentions the number of girls in the class but we dont know the total number of students. Say for e.g there are total 30 students then the probability of selecting a boy will 2/3 but if there are 40 students then the probability of selecting a boy will be 3/4 . Hence not Sufficient.

Take it this way,

Assume - Total number of students = X
$$Then number of boys = \frac{2}{3}X$$

$$Prob of selecting a boy = \frac{number of boys}{total number of students}$$

$$P = \frac{2*X}{3*X}= \frac{2}{3}$$
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Re: In a certain class, one student is to be selected at random [#permalink]

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09 Oct 2010, 14:45
A

1 - 2/3 straight forward answer
2 - noise. no data to derive total class number or boys number also
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Re: In a certain class, one student is to be selected at random [#permalink]

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21 Oct 2014, 15:43
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Re: In a certain class, one student is to be selected at random [#permalink]

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21 Oct 2014, 15:58
In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?

(1) Two-thirds of the students in the class are boys. Directly gives us the probability of selecting a boy. Sufficient.
(2) Ten of the students in the class are girls. Clearly insufficient.

OPEN DISCUSSION OF THIS QUESTION IS HERE: in-a-certain-class-one-student-is-to-be-selected-at-random-136197.html
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Re: In a certain class, one student is to be selected at random   [#permalink] 21 Oct 2014, 15:58
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