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# If a child is randomly selected from Columbus elementary sch

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If a child is randomly selected from Columbus elementary sch [#permalink]

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12 Oct 2013, 12:30
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Question Stats:

59% (02:19) correct 41% (01:26) wrong based on 147 sessions

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If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75

(2) There are 35 more boys than there are girls
[Reveal] Spoiler: OA
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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12 Oct 2013, 12:41
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If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

$$P(b)=\frac{b}{b+g}=?$$

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> $$\frac{b-25}{(b-25)+g}=\frac{3}{4}$$ --> $$b-3g=25$$. Not sufficient.

(2) There are 35 more boys than there are girls --> $$g=b-35$$. Not sufficient.

(1)+(2) We have two linear equation with two unknowns ($$b-3g=25$$ and $$g=b-35$$), thus we can solve for both and get the value of $$\frac{b}{b+g}$$. Sufficient.

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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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12 Oct 2013, 12:49
Bunuel wrote:

$$P(b)=\frac{b}{b+g}=?$$

Thanks, Bunuel.
Could you please clarify how the statement 1 "...the probability of selecting a boy will be 0.75" is different from the question itself "what is the probability that the child will be a boy". I'm stuck here because to me it looks like they provide the same information.
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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12 Oct 2013, 12:51
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LinaNY wrote:
Bunuel wrote:

$$P(b)=\frac{b}{b+g}=?$$

Thanks, Bunuel.
Could you please clarify how the statement 1 "...the probability of selecting a boy will be 0.75" is different from the question itself "what is the probability that the child will be a boy". I'm stuck here because to me it looks like they provide the same information.

(1) says that "IF 25 boys are removed from the school, the probability of selecting a boy will be 0.75"
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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12 Oct 2013, 12:56
Bunuel wrote:

(1) says that "IF 25 boys are removed from the school, the probability of selecting a boy will be 0.75"

Thanks Bunuel! I totally overlooked it.
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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08 Nov 2013, 05:33
Bunuel wrote:
If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

$$P(b)=\frac{b}{b+g}=?$$

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> $$\frac{b-25}{(b-25)+g}=\frac{3}{4}$$ --> $$b-3g=25$$. Not sufficient.

(2) There are 35 more boys than there are girls --> $$g=b-35$$. Not sufficient.

(1)+(2) We have two linear equation with two unknowns ($$b-3g=25$$ and $$g=b-35$$), thus we can solve for both and get the value of $$\frac{b}{b+g}$$. Sufficient.

Hi Bunuel,

Can you please shed some light on why it would not be correct to state the following:

For the statement 1, p(girl)=(g/(b-25+g))=0.25.

Assuming this inference is correct, we can find the number of boys using a two equation,two unknowns approach.

Thank you!
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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08 Nov 2013, 05:48
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Pmar2012 wrote:
Bunuel wrote:
If a child is randomly selected from Columbus elementary school, what is the probability that the child will be a boy?

$$P(b)=\frac{b}{b+g}=?$$

(1) If 25 boys are removed from the school, the probability of selecting a boy will be 0.75 --> $$\frac{b-25}{(b-25)+g}=\frac{3}{4}$$ --> $$b-3g=25$$. Not sufficient.

(2) There are 35 more boys than there are girls --> $$g=b-35$$. Not sufficient.

(1)+(2) We have two linear equation with two unknowns ($$b-3g=25$$ and $$g=b-35$$), thus we can solve for both and get the value of $$\frac{b}{b+g}$$. Sufficient.

Hi Bunuel,

Can you please shed some light on why it would not be correct to state the following:

For the statement 1, p(girl)=(g/(b-25+g))=0.25.

Assuming this inference is correct, we can find the number of boys using a two equation,two unknowns approach.

Thank you!

Yes, it's correct but if you simplify it you'd still get the same equation: $$b-3g=25$$. Thus you'd still have only one equation with two unknowns.

Hope it's clear.
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Re: If a child is randomly selected from Columbus elementary sch [#permalink]

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30 Mar 2015, 05:43
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Re: If a child is randomly selected from Columbus elementary sch   [#permalink] 30 Mar 2015, 05:43
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