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

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Director
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In a certain class, one student is to be selected at random [#permalink] New post 19 Sep 2009, 13:38
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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.

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Senior Manager
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Re: DS = Probability [#permalink] New post 19 Sep 2009, 13:43
reply2spg 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.

Please explain

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

maliyeci wrote:
reply2spg 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.

Please explain

[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
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Joined: 23 Jun 2009
Posts: 360
Location: Turkey
Schools: UPenn, UMich, HKS, UCB, Chicago
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Re: DS = Probability [#permalink] New post 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
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Re: DS = Probability [#permalink] New post 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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