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In a certain class, one student is to be selected at random

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Director
Joined: 12 Oct 2008
Posts: 557
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Kudos [?]: 125 [0], given: 2

In a certain class, one student is to be selected at random [#permalink]  19 Sep 2009, 13:38
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45% (medium)

Question Stats:

50% (01:19) correct 50% (00:47) wrong based on 6 sessions
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.

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Senior Manager
Joined: 23 Jun 2009
Posts: 355
Location: Turkey
Schools: UPenn, UMich, HKS, UCB, Chicago
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Kudos [?]: 96 [0], given: 63

Re: DS = Probability [#permalink]  19 Sep 2009, 13:43
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.

[Reveal] Spoiler:
OA A IMO D

A
1)suff.
since 2/3 of the students are boys, the prob of that a boy selected is 2/3.
2)insuff.
We know the number of girls. But we do not know the number of boys.
Let the number of boys is 20 then the answer will be 2/3.
And let the number of boys is 30 then the answer will be 3/4.
Director
Joined: 12 Oct 2008
Posts: 557
Followers: 2

Kudos [?]: 125 [0], given: 2

Re: DS = Probability [#permalink]  19 Sep 2009, 14:05
Can you explain 1 in detail?

maliyeci wrote:
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.

[Reveal] Spoiler:
OA A IMO D

A
1)suff.
since 2/3 of the students are boys, the prob of that a boy selected is 2/3.
2)insuff.
We know the number of girls. But we do not know the number of boys.
Let the number of boys is 20 then the answer will be 2/3.
And let the number of boys is 30 then the answer will be 3/4.
Senior Manager
Joined: 23 Jun 2009
Posts: 355
Location: Turkey
Schools: UPenn, UMich, HKS, UCB, Chicago
Followers: 5

Kudos [?]: 96 [0], given: 63

Re: DS = Probability [#permalink]  20 Sep 2009, 07:20
it says 2/3 of the students are boy. This can be 20 boys and 10 girls, 30 boys 15 girls or any value that satisfies this 2/3 ratio.

Question asks the probability that a student selected is boy.

if there are 20 boys and 10 girls. Probability will be 20/(20+10)=2/3
Senior Manager
Joined: 16 Jul 2009
Posts: 262
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Re: DS = Probability [#permalink]  21 Sep 2009, 22:24
Agree with A.. As you are already provided with the probability fraction in st1, whereas st2 just provides you the information about the no. of girls in the class..
Re: DS = Probability   [#permalink] 21 Sep 2009, 22:24
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