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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] New post 12 Oct 2013, 11:30
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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
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Re: If a child is randomly selected from Columbus elementary sch [#permalink] New post 12 Oct 2013, 11:41
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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.

Answer: C.
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Re: If a child is randomly selected from Columbus elementary sch [#permalink] New post 12 Oct 2013, 11:49
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] New post 12 Oct 2013, 11:51
Expert's post
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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COLLECTION OF QUESTIONS:
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Re: If a child is randomly selected from Columbus elementary sch [#permalink] New post 12 Oct 2013, 11:56
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] New post 08 Nov 2013, 04:33
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.

Answer: C.



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] New post 08 Nov 2013, 04:48
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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.

Answer: C.



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.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If a child is randomly selected from Columbus elementary sch   [#permalink] 08 Nov 2013, 04:48
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